With a runner on third, Astros second baseman Jose Altuve hit a ground ball in between first and second to score the run. First baseman Corey Hart made a diving stop to field it and tossed to Greinke who raced to first. Greinke touched the bag at the same time as Altuve, so first base umpire Sam Holbrook called Altuve safe.
Greinke turned around and spiked the ball, likely because he was frustrated with the call and/or giving up the run (Carlos Gomez misplayed a ball in center to give Jordan Schaefer a triple to lead off the game). Holbrook immediately ejected Greinke, who had only faced two batters and thrown four pitches.
Milwaukee manager Ron Roenicke came out to discuss the call with Holbrook and was ejected for arguing.
The questionable ejection put the Brewers in a horrible spot because it meant the bullpen would have to pitch almost the entire game. It also came at a bad time for Milwaukee as Greinke is being mentioned in trade talks, and the quick ejection prevented interested teams from being able to see him pitch.
It’s also worth mentioning that Holbrook is part of umpire Joe West’s crew, which is notorious for questionable calls.
Here’s a video of Greinke talking about the ejection after the game. He says he was mad at himself for making a mental mistake on the play: