I Give Up - Why an A-904 Rolls A Sprag?

I studied the stock hydraulic circuits for a good long while, and could not see it happening with stock parts, unless something in the VB was busted, allowing a pressure crossover. Even then,that circuit is dumped straight into the pan, in every manual valve position except reverse. And at such a low rpm, the throttle pressure is very low, and at almost zero driveshaft rpm there will be little to no governor pressure. I would have thought that not enough line pressure could leak into the reverse circuit, to cause this situation.
Hydraulically I couldn't see it.