Defense - soft on the line, badly coached on pass d assignments, linebackers out of position on run defense. Very very un-Badger
Special teams - sucky. Bobbled nearly every kick return, and fumbled one back to the Illini at their own 13, resulting in an Illinois touchdown
Offense - Mertz' replacement got hurt early in the season, so the back-up is a third-stringer or he would definitely get in. Mertz panics easily, gives up on his reads early, just looked bad. Braelon Allen, a potentially great runner, has regressed since his great freshman year. That, in turn, may have to do with offensive line play, which was half-hearted in both run blocking and pass protection. Mertz was under the gun a lot, and Allen ran into a wall on most of his running plays. His back-up, Clemson transfer Chez Melusi, is just average too.
So, long and short, Chryst was doing a crappy job, and after the Ohio State embarrassment, it was probably a gutsy, needed move by the admin to cut bait instead of waiting for the damage to metastasize the rest of the season.