When I go on group rides, we usually sprint to landmarks along the way. I can't win with only one gear, but I join in just for fun, and I have experienced this same thing before. When I started sprinting hard, the front wheel would just bounce all over the place--it's a little bit unnerving. It hasn't happened to me since I switched to bullhorns, so perhaps you should just lean forward more. In fact, I've actually lifted up my rear wheel from pulling up too hard.