I'm curious what started the feathering... mechanical failure?
I suspect we'll need to wait for the accident report to know for sure.
So far, everyone here seems to believe the feather lock control to be a simple safety lock on the feathering handle; I don't know for sure if this is true, but the Reuters report I read seems to imply a rather different situation - that it is a control to lock the feathering section in place physically.
This disctinction is important, because things get tricky at mach 1; the loads on areodynamic surfaces can become very large, and shock waves tend to propagate from prtorusions into the air flow. It is entirely possible - and I have no evidence whatsoever to support this; it is merely hypothesis - that the transition to supersonic flight could cause feather deflection despite the feathering control not having been operated.
So - perhaps we should wait until the experts with the evidence come up with a plausible story...