Without getting into 20 year stuff, it is largely a product of the role of the Prime Minister, who answered initially to the Governor General and after 1956 to the President. Both had complete authority to dismiss the Prime Minister or dissolve their government. Excluding interim governments, of the 16 governments Pakistan has had between independence and 1997, six were dismissed or dissolved in this fashion and another resigned in a constitutional crisis after this power was invoked. Another three resigned in conflicts with governor general. One was assassinated. Two were overthrown in coups. And one became President because he was leading the coup. One was PM for a matter of days before being appointed Vice President. The last one, Nawaz Sharif's third government (1997-1999) was also overthrown in a coup.

All of this is a product of the frankly broken state of civil-military relations in Pakistan. Almost all of Pakistan's presidents have been military figures. The ability of the head of state to dismiss or dissolve Pakistan's Prime Ministers is a feature, not a bug, designed to ensure military control. Even those rare Pakistani Presidents who were not themselves military men have been powerless to stop the military from undermining governments they dislike.

Sourcewise: a good overview of the recent background to Sharif's dismissal and the constitutional provisions that led to this state of affairs can be found this article by Hussein Haqqani (Pakistan's former ambassador to the United States) here: http://www.thehindu.com/opinion/lead/pakistan-sticks-to-a-sad-tradition/article19391743.ece