Killed during tryn ult

Last time I died this game, within 2 minutes of end of game. I'm fighting several of them. I cast my ult, I hear it go off (I know there is a cast time) and 1.5s into my 5s ult rumble kills me with flame spitter, I die and ult is on CD. I have played enough Tryn to know when it should be on, I understand the slight delay due to cast time, but I also know there was plenty of time for the cast time and the ult doesn't go on CD if you die while casting it.