This, plus it appeared as though he may have flown through his own wake? Not much but just enough to get it to roll just a little bit, and once he was rolling, or stalled, too low to recover. When you are low, slow and upside down, it doesn't take much to upset your apple cart.

It seemed to happen right about the same point where he had started climbing on takeoff. He climbed, circled, rolled upside down, and flew in low very near to the point of where he started his climb.

I don't know if there was any wind at the time, but if it was basically calm, there may have been some disturbed air still there. It reminded me of when I would fly the 360's while getting my PPL, and hit the 'bump' at the finish/start point.

I had a friend who died landing a Smith Mini Plane, just after a Steerman with a 450hp motor had taken off. He got into the wake of the Steerman just as he was flaring, rolled to the right, his wing tip hit the ground and he cartwheeled.

And at another airshow (practice?), there was a KC135 that flew throw the wake of the B52, rolled and crashed, at Fairchild AFB I think, back in the late 80's or early 90's.

Thoughts and prayers to their families.