Ultimatum timer thingie

Just did 3 inscribed ultimatums, all to just kill mobs. Above the mob kill counter is like a red bar with a timer. I played all 3 ultimatums pretty much the exact same way and there was no consistency with the timer bar. One of them just hardcore dumped, and i failed it. One bounced around 75% to 50% to 75% and one just stayed pretty much at max. Wtf is this bar based on? anyone know? thanks.

These are the ultimatums with the objective "protect the altar".



The altar is in the middle of the ultimatum. All you have to do is to kill monsters nearby while trying to not let them hit the altar since the red bar is the HP bar of the altar.

OH LOL, thanks