"at that" is an idiom that draws attention to the surprising nature of what's just been said. Here it is pointing out the author's surprise that senior players should be doing such a thing. "pantomime acts" probably refers to histrionic or theatrical behaviour in general; in other words, the players are making a big drama or fuss.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.