Its all about $$$$. Marshall simply can't pay their assistants enough to keep them.
Or Doc has said "this is what I expect from you and our team next season and if you can't handle that then hit the bricks"
The disappointment of this past season leads me to believe that a lot of pressure was put on the assistants, especially the defense side of the ball. I don't look at these changes as necessarily bad things. Rippon was forced to resign. Peterson clearly moved up as an OC. The rest of them moved laterally at best and for not necessarily more money. I think some of them knew they couldn't fulfill Doc's expectations with him on the hotseat and chose to find employment elsewhere.
This is all just speculation, but I'd like to think that Doc is fine-tuning his staff now that he has some experience at HC. He's not gonna come out and say he took out the trash. He likely helped some of these guys find employment elsewhere. Anyone can see that Rippon wasn't the only problem with our team.
The other schools these guys are heading to are in the midst of paying exit and entrance fees. And USM is broke. I don't think they have the money to raise salaries over ours. I think some of those guys are just lucky to still have a job in football.