It was an appropriate and shared expectation. Anger is the emotional signal that tells you there's something wrong, something doesn't match between your expectations and the situation. When anger erupts in us, it is a signal that something is wrong. Something isn't working right. There is evil or incompetence or stupidity lurking about.
Anger is our sixth sense for sniffing out wrong in the neighborhood.
Diagnostically it is virtually infallible, and we learn to trust it. What anger fails to do, though, is tell us whether the wrong is outside or inside us. Do not let the sun go down on your anger, and do not make room for the devil. At its best, anger is the emotional force that drives us to change situations that need to be changed. There are countless examples of courageous changes that happened because people have gotten angry, because a situation has not met with an expectation and the expectation was entirely legitimate.