Why is my ex-boyfriend stalking me?

We broke up almost eight months ago and today he shows up at a bowling match of mine and just stares at me. My friend let me know if he was staring at me. Everytime I got up or moved his eyes followed me. He tried talking to my sister about me but she blew him off, because she knows about all I went through with him. He has a girlfriend who gets around, but hey that’s her life. I’m going to the formal dance with a crush of mine. But him showing up like this really bothers me. I would never take him back, he’s lucky I didn’t press charges against him. I don’t know what to do, I’m slightly alarmed that he’s coming around again and I’m not sure how to handle it. He disappeared after awhile but when I was leaving the bowling alley his car was still there. Any ideas as to why he’s stalking me? Or what I should do about it?