I don't know why, it's entirely possible she was just cheating. But during my last ranked game as surprise party 
, every time 
started her ult channel i tried to stun her with either my Q stun or my ult stun but nothing interrupted it. It happened throughout the entire game and with no way to lock her down she ultimately won the game almost by herself.
Read amumu's ult.
It isn't a stun. It just entangles them, rendering them unable to attack or move.
Kat is already attacking, amumu's ult doesn't cancel channels.
Kinda funny though,
ult doesn't stop ult
But 
solo ulting stops her ult, because it stops channels for some reason.
Amumu's ultimate is not a stun, it's a unique form of CC called an Entangle. It stops movement and basic attacks, but it doesn't not stop the enemy from using spells, nor does it interrupt channeled abilities.
