Since there is no evidence of mechanical failure, even after close scrutiny, I'll propose an alternate theory. I've mentioned this possibility before off-line, out of respect for Sam, but it may be time to make my thoughts public.
IIRC, Bill had fitted a very strong rear brake, actuated by the left hand, AKA clutch, lever. If the motor quit, and the bike was coasting to a stop, Sam's natural inclination from clutch bikes might be to grab a hand-full of "clutch" to extend the coasting distance, locking the rear wheel?