What made him really do this?

My ex and i broke up recently but we would still talk here and there. We hurt each others feeling while trying to be "friends" over jealousy from seeing other people. He started talking to this girl for only a week but i guess they were hanging out almost everyday. She messages me out of nowhere talking sh*t because she knew i was his ex but me and him were not in contact at all while they were seeing each other. I replied few days later we were arguing back and forth and my ex got upset saying i was supposedly ruining their relationship. There was no more contact after that. Until the next day he calls me from ig because i blocked his number asking me to hangout and to "talk". I asked why would he call me if he's with that girl and he responds that he wasn't anymore because they had a big argument. So he picks me up acting normal asking how i been and what not. On our way to the store he parks far away and wanted to talk outside. I was confused because it was so cold, he kept texting and i was just waiting. Then out of the blue i see his sister and his "new girl" coming towards me. They start harrassing me talking sh*t and as i tried to walk away they kept running after me. His new girl pushed me and a whole fight started. They all ganged up on me even while i was on the floor. I'm not sure if my ex was hitting me at one point but he broke up the fight afterwards. I called the cops and reported everything. One random sweet lady had a footage of the fight and i was able to save it for proof. So the whole time it was planned and my ex knew exactly what he was doing. But why all that trouble when we were together for 4 years, had a lot of ups and downs but never this type of situation. I never thought in my life that he would do this to me.
What made him really do this?
Add Opinion