We Find Out Bonnie Pushes Perry Because of Her Abusive Father
This isn't so much a theory as it is referencing what Liane Moriarty writes in the book. Bonnie's motives are a little unclear in the series, but season two is probably going to give us some explanations as we meet her parents (played by Martin Donovan and Crystal Fox).
Still, Bonnie may have pushed Perry because she recognized him from her past as an abuser. I'm more incredulous about this theory since the Monterey community is tight-knit in the series, and she must have seen him before Trivia Night.