Possible reasons behind the killing
Non-commanding position angers General Manzoor
Some claim what acted as a catalyst of the assassination of Zia was the decision of the then Chief of Army Staff Hussain Muhammad Ershad to transfer Major General Manzoor to a non-combatant post in Dhaka as Commandant of the National Defence College. Major General Manzoor was the General Officer Commanding (GOC) of Chittagong 24th Infantry Division, where most of the freedom fighters were placed under him benefiting from the highest posts.
Once the menacing general had been asked to leave, his men simply rejected their (theoretical) allegiance to the Chief of Army Staff and launched a coup. This was all the more ironical because strengthening the 'local chiefs'' power was a result of Zia's will to 'make Dhaka sure for him', by placing most of the senior officers, especially muktijuddha, outside the capital.
Jérémie Codron, Analyst
Major General Manzoor resented a planned transfer to a non-command post in Dhaka. Once the transfer order was sent to him, the coup was launched.
Jealousy
Rivalry between Ziaur Rahman and Abul Manzoor, like most cases of factional conflict in South Asia, had a long past. Even among freedom fighter officers acute rivalries were quite common and publicly known, for example, between Ziaur Rahman and Khaled Musharraf, Zia and Abul Manzoor. Some suspect it was these personal rivalries and jealousies which was a prime motivator for the tragic and alleged killing of one sector commander by another.
Abul Manzoor watched his long-term ambition of being Chief of Army Staff - and potentially, President - fade away first by the mammoth rise of Ziaur Rahman then by the repatriated officer Hussain Muhammad Ershad. This rivalry coupled with the perception of injustice by Abul Manzoor abruptly ended Zia's life.
There has been widespread speculation about the role of Manzoor and his younger officers in the coup attempt. Most of the evidence suggests that Manzoor was concerned that he would no longer be able to resist a transfer to Dhaka, which meant that he, like Mir Shaukat, would be stripped of his command. This would have been terribly humiliating for Manzoor, since he saw himself as deserving of Ershad's position and as a future President. His younger colleagues realised that they might be subjected to forced retirement from the military or some form of discipline if they lost the protection of Manzoor. They all knew that any hopes they might have had for staging a coup would be dissipated once Zia returned to Dhaka.
Cold killing of Colonel Taher increases rebellion
During Ziaur Rahman's ruling the suppression of rivalries and revolts within the army intensified. The BSS and Jashod leaders who had carried Ziaur Rahman to power were systematically arrested whereas the repatriated, less threatening because of their apparent disorganization, became the regime's fresh allies. But his 'treachery' against old comrades like Abu Taher and Abul Manzoor and other notable Muktijuddhas provided ammunition for his critics. For the soldiers who had experienced the Liberation war and 7th November 'revolution', Colonel Taher's arrest, followed by his death sentence, was an act of treason. This triggered a series of unprecedented rebellions.
Even the announcement by 'Biplobi Parishad' after Ziaur Rahman's assassination invoked the ghost of Colonel Taher.
Taher indeed became potentially more threatening, post-mortem, to Zia. The resentment brought forth from the execution resulted in a marked military instability, which inspired chain-reaction mutinies in several regiments. From November 1975 to October 1977, roughly ten serious revolts questioned the military hierarchy. Although none succeeded, all resulted in widespread bloodshed.
Favouritism toward repatriated officers proves distressing for muktijuddhas
In contrast to Sheikh Mujibur Rahman's governance, the muktijuddhas experienced a sharp decline in their involvement in the Bangladesh army and the government roles under Ziaur Rahman. At the time of Zia's death in 1981 it is estimated only 15% of freedom fighters constituted the Bangladesh Army, with the rest made up of repatriated officers (25%) and new recruits (60%).
For these politicised fighters, 'heady' with the heroism of the liberation war, Ziaur Rahman's favouritism towards the repatriated proved alarming. No longer able to shape the destiny of their motherland merely ten years after sacrificing so much, many became bewildered and frustrated. With the loss of influence came the loss of incentives, both financial and social, which were attached to positions these muktijuddhas previously enjoyed under Sheikh Mujib few years earlier before he was killed in such cold-blood. Something had to be done. After all, if the country could survive the brutal killing of their once revered 'jathir jonok' and 'bangabandhu' then who was Ziaur Rahman in comparison, they thought.
These frustrated muktijuddhas were quick to take up guerrilla-style operation against their commanding officers or even head-of-state if necessary.
It seems that Zia's death was not an isolated event, but rather part of a larger conspiracy even though there was no involvement of an external power and the conspiracy was homegrown. It is true that Manzoor was disappointed with his job situation, but this merely served as an "accelerator" for the coup attempt, which was perhaps related to intense discontent among a segment of the armed forces. During the last two years of his rule, Zia had relied heavily on the repatriated and the newly recruited officers and soldiers.
... Zia's policy further alienated him from the freedom fighters both inside and outside the army.
Thus, while Manzoor did have a personal grudge against Zia, the freedom fighters and officers recruited during the Mujib regime had sought to seize power long before 30 May 1981, since all the previous coup attempts against Zia had been led by participants in the liberation war.
Of all the freedom fighters, only General Manzoor had been able to retain an operational command in the army, and this was in Chittagong. Therefore, it may be that he was persuaded by the freedom fighters to stage a coup in Chittagong, after which the freedom fighters posted in different cantonments would mobilise support both within the army and from pro-Mujib civilians.
Few men in history have betrayed the aspirations of their people as did the first leaders of Bangladesh - Sheikh Mujibur Rahman, Khandaker Moshtaque Ahmed and General Zia. When each in turn was called upon to make good, he took the country further along the road to perdition. Once the darling of the independence movement 'in whose magic nae all things are done', Sheikh Mujib as Prime Minister and President became the most hated man in Bangladesh within three short years of its founding. He and his family were killed for it. And the hatred lingers. Ten years after Mujib's death his daughter, Hasina, told me that she could not get the agreement of relatives and neighbours in their home village of Tungipara to erect a suitable monument over Mujib's grave. "People react differently when you are not in power", Hasina said in what could be an epitaph for both Mujib and General Zia. Moshtaque, who succeeded Mujib, has become a by-word for treachery. General ZIa, the next man, was once idolised by the army. But then he showed his true colours and became the target of 20 mutinies and coup attempts in five years. The 21st killed him. As public awareness of the general's real role increases, Zia's memory too has become an embarrassment to his friends.
Anthony Mascarenhas, author of "Bangladesh: A Legacy of Blood" (1986)
Anti-Indian sentiments over Ganges River
Another reason cited for the assassination of President Ziaur Rahman is the bitter dispute over the rights to the Ganges water with neighbouring India.
Bangladesh's long-troubled relations with India, the country that had helped it win independence, was further heightened when the two nations were unable to agree on long-term solutions concerning the lower Ganges River, which meanders through both countries as it flows out into a vast delta of Bay of Bengal in Bangladesh. Various solutions to the water question and territorial claims to islands formed by silt at the mouth of a boundary river were rejected. Matters were not helped much by Ziaur Rahman and his BNP who were commonly perceived to be 'anti-Indian' and 'nationalistic'.
The sovereignty question is particularly volatile: there are hopes of finding oil under nearby waters.
While Zia had pressed India strenuously on the diplomatic front—even sending gunboats to one of the 'disputed islands last month—he was apparently not aggressive enough for a fiercely anti-Indian element with a strong base in Chittagong.
The killers of Ziaur Rahman and his 8 colleagues were apparently linked to these militants.