Answer: The battle of Yorktown started because "In the summer of 1781 it became possible to unite all the French forces with the Americans to strike an attack of the British. The Americans and their French allies decided to besiege Cornwallis at Yorktown. The Siege of Yorktown took place between September and October, 1781." ~https://www.landofthebrave.info/battle-of-yorktown.htm
I will provide you some more links that have more info
Explanation:
https://www.history.com/topics/american-revolution/siege-of-yorktown#section_1
