Mundo can stop Trynda's ultimate

I was playing in aram and fighting against a mundo, i noticed that he could kill me even if i popped the ultimate, probably using the aa after masochism. This happened a couple of times during the match, i died about 2 seconds before my ulti runs out, and i cannot understand why that's happend. P.S. it was TPA Mundo, i dont know maybe this can help