...Why else would somebody [of Pakistani origin] come here and curse him after he died! ....
GS! Simple! People have heard that Sam Mankeshaw was responsible for attack on East Pakistan.
You see! Mukti Bahini, Indira and Manekshaw are treated in Pakistan just like you as an Indian would treat Sant Bhindranwalay, Punjabi separatist movement, and may be Zial huq. If you perceive someone trying to break your country, you won't garland them with flowers won't you!
You should appreciate that many Pakistani with good grasp of Military history are only saying good words for Sam.