I don't recall where but one story from 5 years or so ago said something like all Brians band were paid to tour but one member (I assume Jeff) was paid a retainer. As Brian cut down his touring of late, it makes business sense to review that arrangement. It would not surprise if this was the reason Jeff left.
The potential political machinations involved in guys leaving Brian's band and joining Mike's band aside, it certainly would be far more lucrative to tour with Mike's band, even if it one was paid less per gig.
While making more money (and having more fun playing more gigs) in Mike's band would surely be a factor, I would tend to believe Jeff leaving Brian's band and joining Mike's involves other factors as well. Some of which we don't know.