In 1968, Ayub Khan commissioned a group a West Pakistani economists to find out whether West Pakistan would face any economic problem if they ditch East Pakistan. Those economists gave him report that West Pakistan would be perfectly fine even thrive if it break-up with East Pak. After that report, Ayub was mentally prepared to let go East Pakistan. That's why he was unwilling to hear any legitimate demand of Bengalis to reduce the imbalance. According to Ayub's 1962 constitution, If the President resigns, he should hand over power to the speaker of the National Assembly. But Ayub did not do that, when he was forced to resign in 1969, because the speaker was a Bengali, instead he handed over power to another West Pakistani dictator, Yahya Khan. His mentality was shared by a lot of other military and civilian bureaucrats of that time who were willing separate from East Pakistan.
Pak military leadership wanted to ditch East Pakistan, but did not want to be accused as the one who broke Jinnah's Pakistan. So they carefully orchestrated a conspiracy where they enticed Bhutto to agitate and create a political crisis. They were also determined to teach Bengalis a lesson before the final break-up, when millions of refugees will flee to India, India will be forced to intervene and snatch away East Pak. Thus absolving their culpability. That's why they started to kill indiscriminately Bengalis, especially the Hindus. 10 million refugees fled to India, 9 million of them were Hindus. Now Pak generals were satisfied that Indian intervention will come.
But they became impatient when even after 8 months, India was dragging it's feet, so to hasten the process, they made a pre-emptive strike on Indian airbases in the Western side. Earlier they removed Tikka Khan and appointed AAK Niazi to take responsibility of defeat and surrender. AAK Niazi was a boastful, arrogant and less intelligent mid-level military officer whom they choose to sacrifice in the alter of their conspiracy. No higher-ranking general was willing to take charge of Pak Eastern Command. So things happened as they wished. Common Pakistanis now blame Bhutto for the dismemberment of Jinnah's Pakistan and AAK Niazi for defeat and surrender, Pak military remains as innocent as ever.