Burning propellant ejected
This was in an Estes Indicator. It was the very next flight after a CATO of an A10-3T in my Checkmate.
This one popped just as the rocket neared the top of the launch rod. Flame shot out of both ends of the rocket and the motor shot out despite being held in by a motor hook. My family members again saw a fireball go skyward out of the rocket.
This one is the same as the Checkmate failure - the inside of the motor, while very black, is remarkably "clean". I had other A10-3Ts launched this day that looked different after use.
Fortunately there was no damage to the rocket.