My mothers shelty did something like that. We had the windows down in the van, driving down our street. The dog saw the house, got real excited out of nowhere. She thrusts into my lap and flew out the window. Bad timing, she went out as we were passing a telephone pole, mom tried to slam on the brakes, but it was too late. She hit the pole and fractured her hip.
Still have no idea what possessed the dog to do that. She never did it again after that either.