It's not hard to imagine why a coach with loyalty to the players he's been with for a while might clash a little with the new staff demanding excellence. Or why stress might build up in those circumstances. Pure speculation. It could be differences in opinion over scheme as well, but I have a harder time believing that - I think that issue would have come up a while ago.
It could also be strictly personal, or some combination. Regardless of the circumstances, I wish Lou the best. If he's back after 3 months, great, if not I wish him the best. I'm not concerned about the defense, Mora knows what he's doing there. I wasn't expecting a monumental leap from last year anyway.