Yu Darvish had retired the first 17 batters he faced (that's like a 1-2-3 inning against non-Astros teams!). He got what he thought was strike three against Jonathan Villar to end the sixth, but home plate ump Ron Kulpa didn't see it that way.
Kulpa—already hated by Rangers fans for a blown call in the 2011 World Series—called it a ball, and Darvish would go on to walk Villar. Catcher A.J. Pierzynski exchanged words with Kulpa, and was ejected for his troubles. It's not clear what Pierzynski said—if you take a guess, you're probably right—but Kulpa was definitely not doing anything to defuse the situation.