en.wikipedia.org
Bret should never have hired Bobby Hauck to be his D coordinator. Bobby Hauck was a briefly retired FCS head coach who hadn't been a defensive coordinator since, wait for it...ever.
That's right, check Hauck's resume. He'd been a defensive position coach, special teams coordinator, and head coach. He'd never once been a D coordinator in his entire career prior to joining Illinois. Additionally, he hadn't coached above the mid-major level since being in role at Washington that ended in 2002 (forever ago).
Hauck was a solid FCS head coach in his two stints at Montana, and he nearly won it all there multiple times.
Hauck was never a good fit to be a FBS Power 4 D coordinator, and Bret was a fool for ever thinking it would work.