Surprise, surprise: like all of Shakespeare's tragic heroes, Coriolanus is his own worst enemy. The qualities that makes him a war hero lead straight to his downfall back home. Turns out that aggression, violence, and harsh language aren't as effective on voters as they are on enemies.
The Tribunes are the guys who are always talking trash about our hero and work to get the plebeians to mutiny against Coriolanus. They're telling us they think Coriolanus has way too much pride and is a big fame monger, so they convince the plebs to take back their votes during Coriolanus' campaign for consul. Sounds pretty antagonistic to us.