Why are bosses Enhanced?

I noticed when I went into Simulated universe the first elite boss I went up against was a voidranger: Trampler. It went into the arrow attack repeatedly and so I thought I'd check it's status and showed it was enhanced. I don't know what causes it to be enhanced. Was it something I did or is it a random thing in SU? I thought I'd try to look it up but no info on it anywhere.