I've found one other time it has happened. It was last season and the player involved was Taylor Walls of the Marlins. The umpire was ...... Nic Lentz.
I guess it's similar to drawing a line on the ground with your bat if you don't like a call.
Big difference between Walls and Contreras was Lentz was the home plate umpire and Walls looked at him when he tapped. Contreras never looked at Lentz and was most of the way back to the dugout, which means Lentz was watching him the whole way, looking/listening for something. for something. He wanted to toss him.
Lentz will be behind the plate tonight. I think every fan in attendance should tap their heads on every ball/strike call.