I noticed during both Fri and Sat games that the Brewers catchers would come set and flash receiving position with their glove in one location (like they were calling for a pitch up or a pitch in) prior to the pitch being thrown and then once pitcher motion began would move to second location where pitch would ultimately be delivered.
So my suspicion is that Brewers players (Sanchez, Adames, etc) were either (a) peeking back at McCann to see location, or (b) getting signal from runner on base. I think it was the former that set McCann off on Adames because he saw stealing a peak back at the glove. Some of this could be mitigated if McCann actually called better pitch mix and moved in/out….to me McCann was basically setting up outside almost exclusively and would just vary up/down. And he would very early set his target for the pitchers.