That's just the whole backwards thing, you might not realize it, but most likely as your on the lip you sort of freak out (it happens sub-consciously sometimes) and try to turn around to face forward too soon and catch you tips. You just gotta relax, and be extremely comfortable riding switch. Also, practice switch spins on rollers and knolls, they're much lower consequence and can help you with your timing on the take off (especially cause you have to pop to make it happen, so you'll get a really good feeling of the pop). The spin itself is much easier than forward spins.