It's not ABNORMAL, but it's not a very common thing to do. From what little information I've been able to dig up, it doesn't seem to serve any real purpose. It's apparently just something that some BP's do. It's entirely possible that ALL of them will do it occasionally, we just don't see it most of the time.

It's a little funny looking, but it's not as amusing as tail wagging. Some of them do that too, and it's hilarious.