Coaching is probably one of the more demanding professions. If you have a chronic medical condition, it is going to make it worse in most cases because, by definition, it's hard to 'take care of yourself' with the schedule it demands. Add in the stress of 'win or you're out' (true for the vast majority of coaches) and that's going to be a challenge.
I think he took a break and feels good again and may want to prove he can do it at the NFL level (or is looking for one last payday).