It's a bit hard to imagine incapacitation leading to that sort of flight profile unless it was a seizure or something causing a total lockup on the controls. I'd think even with someone slumped over the stick (which doesn't seem very likely with a Cirrus sidestick anyway) the aerodynamic forces involved would eventually shallow the descent out a bit. You would assume he would have shown the kids how to operate the parachute too in case he suddenly keeled over.