Zeth's one of those gangsters who's obsessed with being respected, and he takes it as a gross personal insult when someone keeps him waiting. Especially in the middle of a meeting.
In the first draft of the script, Smiley was also coordinating another caper that was taking place at the same time, and was getting phone calls about that situation as well, so he ended up stepping away from the table to take a call not just once but three times––and Zeth got madder and madder each time.
In the end, that second simultaneous caper, and the other two phone calls, were cut from the scene for the sake of keeping the main story moving forward. Fortunately, there are plenty of other ways to infuriate Zeth. It's important to keep making Zeth angry because we love how Mike draws Zeth's angry face.