Okay, I figure it is Monday somewhere in the world by now, so I'm answering the question.
When I read the prologue, I assumed it was the chick, Marlena, that did the deed. She was the apparent antecedent to the she in the descriptions, and there was no reason to think otherwise. I did assume that I would learn more about it later.
As the story unfolded, there were a few occasions where Rosie had the stink-eye for August, and seemed to have both motive and opportunity. I found myself thinking "why didn't she just stomp him then and there?"
Oddly enough, it still didn't occur to me that she would turn out to be the one. When I got to the chapter where the full story of the stampede is told, It took me a minute to realize "hey, wait a minute, I've read some of this before" and then flip back to the prologue and say "ah, sure enough." I even checked for any discrepencies between the two versions, but I think Sara Gruen, the author, must have just done a copy, paste, and edit.
I will say that although August is not mentioned by name in the prologue, it did not take long to work out who "that son of a *****" was. What a psycho!
Steve