I found the patent and other document descriptions of the unlock system disturbing.
Pneumatic unlock actuator?
I can understand the actual feathering being pneumatic, but the unlock? With all the simplicity of the design, why not a physical cable to rotate the "hook"?
Hell, at apogee, I could even see a cable system to move the feathers. Zero aero loads and simplicity. Hardest thing would be moving the feathers once back at 60,000 feet or so.
My prediction is that there was a physical problem with the deployment actuators that allowed the feathers to move once they were unlocked. As I mentioned, our Viper c.p. moved a significant amount in the transonic regime, but then essentially stopped moving aft at M 1.4 Who knows, waiting until M 1.4 may have helped keep the feathers "streamlined".
The NTSB statement "of fact" that the feathers deployed needs to be clarified, ya think? Did they simply move on their own or did the pneumatic actuators move them?