I don't at all consider myself an 'expert' or 'in the know' on these issues, but just from stuff that I've heard from other or seen as 'public knowledge' Carl had to leave because of issues, Bo basically let him 'control' the defense to help him get that appeal for a HC gig. Bo and Ekeler never got along or saw eye to eye, him leaving was a mutual decision. Sanders, like Carl, had issues that he had to leave for..seemingly a little more 'urgent' simply by the way he departed. Raymond and Bo didn't seem to get along that well and, if I recall correctly, Raymond was the only coach to not receive a raise after this past season...
As far as the "all the coaches leaving" thing, I don't know for sure, but it seems like a play off the Wisconsin coaches thread a bit....
Ekler...was the LB coach at NU and left for Indiana to be the DC...no surprise there.
Watson/Gilmore...encouraged to "move on" after it was clear they were coasting.
Sanders...off the field issues/behavior caused his ouster.
Carl...left to become a head coach.
Raymond...I guess he couldn't turn down his alma mater after all.
So while Bo has had several coaches leave none of them "left" because they couldn't "get along" with Bo.