I think others, including me, have given Grantham the pass because he wasn’t coming out in the preceding weeks talking about how he was committed to the program, staying, etc. Also, coordinators are a dime a dozen as far as turnover - they’re out there trying to survive in the slim chance they actually breakthrough to HC if they desire it. The guy is a journeyman coach - albeit a successful one.
As far as why he would leave now - I don’t think it has anything to do with Moorhead so much as it means I think he’s found a solid chemistry with Mullen and he feels comfortable with teaming up with him in Florida, possibly for the long haul before maybe ascending to a head coach spot one day.