Looked to me that as he rolled he tagged zero G for a second, and the engine coughed right before the plane was stabilized inverted. It was still descending, never quite leveled off inverted. From the control surface positions, it looks like he tried to abort but had no upwards vector and pulled the stick nearly full aft before he rolled to less than 90 deg bank, so the nose dropped fairly rapidly with a high sink rate. Basically he aborted the inverted pass but did not have the necessary altitude to do so at that time, and hit the ground still attempting to roll back upright. The elevator is pretty far up so his roll rate may have been reduced by pulling it into a stall during the attempted rollout.
There could have been a zillion other things going wrong that we can't see from the video. It didn't look like he was using much rudder during either of the rolls, even though that could have helped with the roll rate or to keep the nose up, decreasing the sink rate. He didn't pause inverted to push the nose up before the abort, which tells me that he felt ground collision or some other catastrophic event was imminent and worth the risk of the abort. I don't think he ever arrested the sink rate caused by the initial roll to inverted so they were doomed the instant he decided to abort the inverted pass.
My best guess would be the engine sputtered a bit and he aborted prior to ensuring he was climbing away from the ground, resulting in too much altitude loss during the attempted rollout. Pulling the stick too far back too soon may have aggravated the situation but they were already dead the instant he started the rollout before getting an upward vector away from the ground.
