Apparently a consultant back in 2014 was taken on despite warnings of domestic violence, sexual assault and abusive online behaviour. When face with this MM asked for information and many who had dealings with the consultant emailed Mike on their personal experiences with the consultant. Apparently Mike took the view that things were exaggerated or made up only after forwarding the emails to the consultant. Mike of late made a statement that the consultant was only a play tester and that WotC has stopped dealing with him. The consultant known as Zak S has a credit in the PHB as a consultant which is separate to play testers mentioned. Many were upset with Mike trying to low key his part in this and for him failing to take any personal responsibility. His twitter account with the message and the responses is probably more enlightening.
My understanding is that the allegation that he forwarded the emails to Zak S is not actually supported by anything. All I've seen is a screen cap of Mearls asking for evidence of the allegations.
Let's not join a witch hunt without some kind of actual reason, guys.