The high priest just lost two sons. They didn't follow instructions. Just as their father and Hur didn't when they kept Moses hands up so the people Joshua had picked would stop dying.
Amalek attacked because besides, Moses and Narchon, nobody really could stand against him, since they had all betrayed Josef, even when leaving Egypt.
This was embarrassing watching his sons repeat the mistake he made and pay such a price. Did Yehuda have to go through this?
The three day fast during Pessach in Shushan was absolutely required, make no mistake. Ahron and Hur owed complete obedience and faith in Moses, all the more so us, even if we pass "unnoticed".