I agree entirely that some wouldn't have happened and came from frustration, but the behaviour happened and deserved punishment.
There was a lot of touch and fall going on towards the end, but a lot of them were fouls, and obviously so, even if they were played for. Young and Pogba aside, I don't recall too many bad challenges from Utd players. Lukaku was booked for a stupid trip on Kompany, I forget what Herrera was booked for.
There is grey area of booking when advantage leads to goals, as happened a couple of times - Matic got away with a deliberate block.