A partially broken spoke can cause spectacular things to happen if it were to get wedged against the fork. The telltale indicators of such an accident would be gouges/scratches on the back of the fork where the spoke would have to have jammed.

Ordinarily, a broken spoke will pull a wheel out of true, and it would be like you hit the brakes with moderate pressure. Given the lack of clamping force from the opposite caliper, there's really no way you could rub a brake pad hard enough to endo. Something else is needed to stop the wheel that quickly. If an inspection of the fork turns up damage on either the back or inside of the fork, its a possibility something jammed the wheel.

I'd bet money that something else caused the crash, and the broken spoke is a result of the crash not the cause.