Rich is correct, you are indeed throwing a "curve ball" Nice cast to have in your arsenal but maddening when it throws off your accuracy.

The flyline follows the path of the rod tip when you stop the rod, you right hand is turning ever-so-slightly to the left.

Practice 25 foot casts to a target until the line unfurls in a straight line. Think about driving a nail straight into a wall.

Joseph

.....or stand a little further to your right.