Why is my ex boyfriend being a jerk?

So it's been about 8 months now that me and my ex broke up out of a 2 1/2 year relationship but it wasn't a clean break up either. So we just came off Summer break and i finally see him again, I thought we would finally forget each other and go on with our own lives. I have an amazing boyfriend right now who i've been dating for 4 months and I guess he had found this out, & today I was walking down the hall way and he yelled out tramp in front of all his friends. Like what's the need of it? I don't bug him with his life. He also ended up being in the same cl as i and was complaining to his friend that i was in it, then he switched out the next day. Help me with the understandings with his immaturity? why is he being dumb.. i also wanted to mention he has a gf now that goes to college who he has been dating for 6 months.