Why is my ex boyfriend causing drama about me with his girlfriend/my ex friend?

Me and my now ex were in a really serious relationship and we were even gonna get married next year and I loved his parents, they loved me, and he broke up with me on March 7 and the next day he went back to his ex girlfriend who used to be my "friend" who has cheated on him numerous times, they're still together even though he claimed that he's unhappy with her and she shows some signs that she's unhappy too but they still won't let go. He even lies to her about his flirting just to stay with her. But ever since they got back together things have been really rocky between me and each of them. He hasn't been treating me right and he left me and went with her, he tries to keep me on the side as a girlfriend while he's with her. And it's not good for me to talk to him so Saturday afternoon I decided I really need to really try to avoid him altogether. So I stopped calling him and answering his calls, and when I see him at the workshop like on Tuesday and Wednesday I just try to do my work and talk to people who aren't part of it. And he kept giving me this rude look, looking at me and making rude comments to me and him and his girlfriend are creating drama about me and talking shit about me. I'm assuming it's because I started ignoring him and avoiding him. But I still don't get it. Just why why why? He made his choices and he wouldn't make up his mind and pick one girl so I had to start making my choices by avoiding him altogether, so why is he doing this with his girlfriend now?
Why is my ex boyfriend causing drama about me with his girlfriend/my ex friend?
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