After everyone's response (thanks very much, by the way) and a little research in the ASA rule book, I think the PU invoked R4.10 since he too judged it as a "wreck" and not OBS or INT:
During a live ball situation, when a player becomes injured, and in the umpire’s judgment requires immediate attention, the umpire shall call “DEAD BALL”. EFFECT: Award any bases that would have been reached.