My guess is the alleged "defensive guru" we hired as HC wanted to change some things we do on defense 'while' LeBeau still hung his name on it. In the old school world...that ain't gonna happen.
Dick would rather resign than allow a guy who has exactly ONE year coaching a defense ON ANY LEVEL (college or pro) put his sophomore fingerprints on his 33+ years of being a DC.
I wouldn't allow it either. If Butler will, he probably gets the job.
You might want to go over to the "Paper That Shall Not Be Named". Tomlin already started putting his finger in the pie "after" DL's defense was getting the **** tore out of it at the beginning of the season. And "lo and behold" the defense got "better" (definitely not great) after he did so. All this 'while' Lebeau still hung his name on it.
So maybe DL got pissed about that. He sure as hell couldn't "do" anything about it other than resign because the alternative was raising a fuss and getting fired by his boss; the Head Coach. Maybe Tomlin told him it wasn't going to change back to the old way and DL decided to walk. There's a lot of maybe's to this and as someone said, we won't ever know for sure. What we do know is that DL's defense was turning to crap 'while' he still hung his name on it. Whether that was due to talent, the scheme needing tweaked or a combination of both is open to debate.