Very strange. I have been thinking about the fact that Bill O'Brien most likely also wanted a chance to be the next guy after Bill here if Bill chooses to retire in the near future and didn't want to be automatically blocked from that before he even started by Mayo. I wonder how things are balancing out.
Perhaps it's less complicated than we all assume. Mayo didn't really want to leave the area or the team, he just wanted more opportunity to grow his knowledge (and a lot more money). Kraft gives him a hefty salary increase and promises him more Belichick time but perhaps it's not quite the "heir apparent" situation we've all discussed. That's not to say that he WON'T succeed Bill, but at least it lets O'Brien also try his hand at candidacy too.