mobs taking damage on contact

I am not sure if this is a just a setting, but I am pretty sure it's a bug because I don't remember this happening last time I joined the world. whenever I walk into a mob (in creative mode at least) it takes heavy damage. This does not affect the Ender Dragon. Is there an explenation for why this is happening?

 

update: it works in survival mode. I can easily kill mobs now.