ZED time question

The triggers are good shots to the mobs... like when I shoot once with the shotty and kill 5-8 mobs at one time that usually triggers a ZED time. Also when using pistol and random busting at first, then aiming for a domer *headshot* and you get one that sometimes triggers it as well. ZED time is a random factor that for the most, which is triggered by good shots or some kind of cool even that leads to mobs deaths