Markakis was ejected for the first time in his career during Friday's 2-0 loss against Texas. Nelson and Markakis began having words during his sixth-inning at-bat.
It escalated after Nelson called a strike on a 1-0 pitch from Texas starter C.J. Wilson that appeared outside.
Nelson then called the next two pitches strikes and threw Markakis out after the two started arguing following the final pitch.
Showalter then got tossed in the ninth inning after offering some of his opinions, but said it's a new day.
"Turn the page," Showalter said. "I only take the [lineup] card out the first day, probably [will] take it out today. It's a tough game to umpire. The game moves so fast sometimes."