Rocket motor not the cause, it did not blow up.
The aircraft suffered an inflight breakup from aerodynamic forces. At this time, the belief is buffeting caused the deep stall tail configuration to prematurely deploy at too low an altitude. It should have still had the mechanical locks securing it, but the co-pilot disengaged them too early.
Much like lowering the landing gear on the Space Shuttle while it is being launched.
"What's this button do again?"
No don't touch tha..."