Bosses that randomly target people with spells will run after those people if they try to leave before the fate is finished. Anyone who fate grinded while the lightning fate was going on would've seen the dreadnaught/chaos demon chasing after random people from a mile away.
Player x sees fate boss is almost dead.
Player x sees that another fate is up.
Player x sprints off to do the other fate.
Fate boss happens to target player x with Void Fire II.
Fate boss dashes after player x in order to get in range for the cast.
Fate boss reaches boundary and gets yanked back.