Hiya,
My Dotty does something similar to your little one.
We've had her since May at 16 weeks and as soon as we got her home and settled her in, most times when I picked her up, she'd kick her legs out either side and to the back. It's really quite comical.
When I lift her up, her back legs are as wide as they can possibly be and she often kicks out when trying to catch her, lol. She walks with no probs, so whether it's one of those things that some piggies do, I don't know. Funny though, isn't it, lol?
Beth xx