Question

What reason did cornwallis give for staying at yorktown and trying to defend his position there

Answer 1

A sudden tropical storm disrupted an effort to move his army northward across the York River to Gloucester Poin.
Hope this helps

Answer 2

The correct answer to this open question is the following.

The reason that Cornwallis gave for staying at Yorktown and trying to defend his position there was that he had received orders from General Clinton -the commander of the British troops in the Revolutionary War-  to protect the harbor in the Chesapeake Bay, to guard the British fleet. Lord Cornwallis chose Yorktown because it had deep waters in the York River. But by that time, the Continental Army led by General George Washington had already received the support of 5000 French soldiers that helped the Americans to defeat the British.