Paul - what a horrible tragedy for that man's family, especially that they watched it happen.
Probably wasn't a "stall" in the airplane sense, as the spinning rotor never loses lift. But if the pilot somehow gets the nose up into a steep angle, the lift isn't enough to keep the gyro from going down. And as you say, if it's at low altitude - recovery may not be possible.
Like many small trainer aircraft, gyros are essentially stable, and will recover to level flight by themselves, even if the engine quits. But they need altitude to give them time to recover.