I think at some point in a battle, Derpy either temeporarily hurt her eye or lost her sight in that eye all together. Either way, she wore an eyepatch, and discovered that now her eyes were not crossed she could fly straight, and really well too. From then on she permanantly wore an eyepatch.
Wow this looks amazing! The characters have a really good pose and are impressively showlike. Love the attention to detail on the smoke trail, and Ditzy just looks astounding with that eye patch. As said, general Ditzy is best Ditzy.