In fact what triggers zed time?
Zed time, or Dramatic Events as it is called in behind the scenes, are random triggered on every zed kill - but they also trigger from a few key events.
Killing 4 or more zeds with one LAW rocket, Pipe bomb, M79 or frag nade = 5% chance.
Killing 2 or more zeds with one LAW rocket, Pipe bomb, M79 or frag nade = 3% chance.
(M32 does not have this extra chance...)
Headshot that also instant kill the zombie (no bleed out) = 3% chance
Zombie killed by commando or berserk during a dramatic event while having extra zed time extensions = 100% chance.
Chance to trigger dramatic event on every zombie kill when zombie die within about 3 meters away from the killer = 5% chance (if not already triggered by killing headshot or AoE multikill)
Chance to trigger dramatic event on every zombie kill when zombie die more than about 3 meters away from the killer = 2.5% chance (if not already triggered by killing headshot or AoE multikill).
Winchester (LAR) also have code snippets that indicate it could have extra chance to trigger dramatic events, but it was never fully implemented.