My guess is the canopy popped open.  The guy could not reach the handle over his head so he disconnected his seat belt.  He raised up in the seat to grab the handle.  The aircraft pitched down with negative G and out he went…  Just speculating.

Steve Colwell  Legacy

I should have said, this should not have happened.  There are checklists and another instructor in the cockpit that should have caught seen the no seat belt.