He left because the future is bleak. It isn't necessarily because something is happening in the background, but there are too many challenges and politics involved fighting little battles with Wasps/ ACL.
The consensus is that working at CCFC is extremely hard work - skeleton staff, being asked to do more for less, politics, uncertainty, lack of resources, lack of leadership.
Think AB is one of the sensible ones that sees life in the bigger picture and that he can do a similar job elsewhere without any of the headaches.
There is no conspiracy with this, apparently he'd just had enough of working in such difficult circumstances.