Why do I get angry when people do things I wouldn't expect them to do?

because you think you know what everyone will do. The answer here is to actually expect them to not do what you expect from them and youll never be disappointed at all because people will almost do the opposite of what youre expecting so quit expecting it

It's a human mentality...at those conditions just try to be calm because when a human gets anger he/she doesn't think about the next step he/she might going to do.