I can't really say. Maybe it was struck by lightning... It left the rail straight, and got up maybe 50 feet, then went into a corkscrew. This was the Apogee kit, so it wasn't a small model. I think the owner said it was an H-motor, but I don't recall who's or which one, though he said it's flown fine on that motor before.
I was looking at EXPjawa photos and it looks like there was a reduction or reduced exhaust flame from the motor. Maybe there was a partial blockage of the nozzle that caused the rocket thrust to be diverted?