Sometimes, when you have many or powerful towers with ability to teleport creeps back in time they come to a point where where they cannot be teleported further back.
The easiest way to reproduce this is to play a game with a carry placed towards the back of the map and place Mr Iron closer to the start. Let Iron eat all of the attack speed and luck items than equip him with 2 items from withered set. He will be capable of continuously holding some creeps out of his range until this bug occurs.
I guessing this is a bug, but it could be intentional not to make withered set on Iron too powerful ;).