No, that's a new one on me too. I'd be pretty freaked out as well.
Maybe its happened to one of the goshawks I have raised while I was blissfully ignorant of it. I have never tried to put an eyass goshawk on its back, so I don't know if they normally can right themselves.
My then 2 year old daughter might have when I raised my first one, but if she did none of us noticed. She thought that goshawk was the best toy ever and was always [gently] playing with it.