The reason for the attention: Alex Sanabia.
As the camera was on him, filming his reaction to giving up a rome run to Phillies outfielder Domonic Brown, the Marlins pitcher pretty clearly spit on the baseball before pitching to the next batter.
Realistically, he probably wasn't trying to do anything illegal. Still, it was clearly against Rule 8.02 (a)(2):
In case you haven't seen it yet, click here for the video. Also, here's an animated gif:
What do you think? Fair or foul? Should the MLB investigate this? Or just let it slide?