Seriously one of the most questionable calls I've ever seen. They had everything going at that point and a complete momentum swing.
Going for an onside kick outside of end game situations means one of two things, you don't respect your opponent and expect to clobber them or you don't think you can win without getting a possession advantage so you're willing to take the risk.
If Frost thought either of those two things he's an absolute idiot. I don't think he thought either of those and think he is just simply a bad coach at this point.