"My guess would be that this may have been standard procedure and perhaps the design of the craft was such that it was considered a safe procedure do as it was a 2 step process. Or maybe not and the pilot acted prematurely. Or this WAS the test."
Until we know how much time was left of the rocket burn, then we can't really guess.
They were transitioning through Mach 1, the feather speed at Mach 1.4 and the end of the rocket burn (and thinner atmosphere) may have only been another 2 or 3 seconds away, so unlocking the feather control in preparation would have been reasonable. Acceleration when pushed by a rocket motor is pretty brisk, I understand, so a good pilot will be trying to "stay ahead" of the aircraft.
One test pilot lying in hospital will have the answer.