There was a long thread about it here when he left, with lots of rumors. IIRC, they were all about sexual misconduct of some sort, but nobody mentioned anything vaguely "rapey" or even "assault-y." Instead, the gossip was that he'd hit on or slept with women he wasn't supposed to hit on or sleep with.
Brooks enthusiast wrote:
So why did he leave the Brooks Beast team? It's a lonely sport without a team/training partners..
The consensus was that if he'd done anything really bad, Mackey would not have agreed to keep coaching him.