I believe he slipped on something a fan threw into the ring that caused him to drop Droz on his head. Definately tragic.
Yeah, I seem to remember Droz in an interview claiming that a fan had thrown a drink into the ring and the liquid caused D'Lo to slip.
There are also some claims that Droz's baggy shirt caused problems for D'Lo holding onto him properly during the move.
I remember reading Foley's account of the incident in his book: he said D'Lo felt so damn bad about it, while Droz - lying on a stretcher - told him not to blame himself; that accidents happen.
Droz sounds like a classy guy.