It might look easy being a member of a baseball park's grounds crew, but that tarp is much heavier and more cumbersome than you think it is. It's windy, it's wet, and the ground can get slippery -- sometimes, that results in someone loosing their footing, and, I don't know, maybe being dragged underneath the tarp, never to be seen again?
In the moments immediately after, it's unclear if anyone is aware that one of the grounds crew members is stuck underneath the enormous tarp that is now being weighed down by rain. Someone runs over to take the suddenly vacant spot to help them get it into position, and between that moment and the GIF below, it's realized that maybe they're a man shy of their full roster.
You can see the little bump traveling, like a kitten moving underneath bed sheets. He's not a kitten, though: He's a totally embarrassed dude who slipped and fell on television and got trapped under a gigantic tarp.