User blog comment:A Wikia Contributor 9337/Heavy build explanation/@comment-123.211.182.164-20160115044434/@comment-24659969-20160115092806

The cooldown are different between one zombie type to others. Creeps and normal weak boss such as requrg only have 0.1 second CD before they can be stunned again. Higher level such as apex elite runner have much more like 0.25 sec CD and a bit of resist (so lv 1 concussion only results for 0.3 stun instead 0.5 for example)

This system existed to prevent a heavy class from perma-stuning a target.