This is said in response to the listener having expressed their annoyance at having to wait. The speaker is surprised that the listener is annoyed because this person does not usually get annoyed about such things. Usually they are prepared to wait.
"You're usually very patient" -this is not how you usually act in such situations
"so why are you being so unreasonable about waiting five more minutes?" - I would like to know why you are impatient on this occasion it is not a big delay
Hope that helps
Student or Learner