One who likes to push buttons
. That is, a person who enjoys getting people irritated and starting fights and arguments; a colloquial term for an instigator.
Not to be confused with button masher
Sarah: Ooh, that Steve is such a dick! He keeps making fun of my weight even though he knows I'm sensitive about it. Kate, do you think I'm fat??
Kate: No, no. Don't worry about Steve, he's just a button pusher.