Why do coaches leave there alma mater all the time?

So I have noticed a trend running mulitple dynasty modes. Coaches who are already at their alma mater and doing great leave to go to a worse team in conference which makes no sense. Can we please make an update for a future patch that if your at your alma mater and can win there is not reason you would leave. I mean sure maybe for Notre Dame or something but here is a great example and it litterally happens every time I run a dynasty.

Sitake at BYU - undefeated and its his alma mater - yet every single time he leaves to go sign with Oklahoma State which is a definate step down and again why would he leave his alma mater when they are winning? it makes no sense. 

Just saw Vandy coach leave for Missouri - same problem