There are several causes for the decline of the Mughal rule in India.
An open country was a pre-requisite for success of Mughal army, for without this their cavalry could not be deployed effectively. In a mountainous terrain, they were at a terrible disadvantage.
The army became, in effect, a body of mercenaries who served for personal gains, and were ever ready to desert when things went bad. There was no discipline in the military.
While Mansabdari system was the key to the structure of Mughal administrative setup, it had its own disadvantages in the army. Each man had so much to lose and so very little to gain, for example, a cavalier rode his own horse, and if it was killed in action, the compensations were disproportionate.
The most critical point of a battle was the death or disappearance of the leader. The troops lacked discipline and determination; and were subject to panic and sudden flight.