Yep, mine do it too. I have quit fixing them, I figured they will fix themselves eventually and I don't mind the flyaway ears so I don't worry about it.
I swear Abbie makes her ears go inside out on purpose. She rubs the side of her head on something until they turn inside out. I thought at first her ears were bothering her or something and had the vet take a look when she went in for her after-spay checkup and she said they looked fine. If you fix them after she turns them inside out, she will often go rub them again and turn them inside out again. Maybe it feels good to air them out? Who knows, they are just silly.