If the jury bought the idea that she may have drowned accidentally and all Casey was doing was covering up out of panic, then there would have been no abuse, and the jury's decision makes sense. Still, the duct tape is pretty hard to explain away in that scenario.
I'm not sure how the jury could come to that conclusion though since there was absolutely no evidence that's what happened. I'm not sure how the law views it, but in my mind the jury should have been instructed to disregard the entire notion since there was no evidence at all presented to suggest it - nothing even remotely presented that sets it up as a plausible alternative.
I suspect casey's Dad is what presented the biggest reasonable doubt. He tried to kill himself and left a note, while vague, still expressed personal remorse as if he had some responsibility for what happened.