Why would they act this way?

My bf and I have been together almost 2 years. He dated a girl a few years ago for 7 years. They have a lot of common friends. They broke up because he found out that she was cheating on him/was taking drugs/living a double life. She started dating a new guy a month after they broke up. He was devastated. At the beginning all their common friends took his side. His very best friend told him that they should break up/helped him move out of her house.
Two years later when he started dating me, she asked if they could all be friends again (what a coincidence?). The common friends funnily enough do not support our relationship at all (e.g they always like her photos of her and her bf while they never like the ones he posts of us and liked the ones that I am not in, friends that were initially nice to me that changed and ignored me when I was at parties- pretended like I wasn’t there). They act as if I am the reason they broke up/it bothers them when I am around. It kind of bothers me even if it shouldn't. His best friend even started dating my bf ex’s best friend and now hangouts with her/likes her photos when he was the one who told him to break up with her. It just does not make sense. While we were all out at lunch one day (we were 2 couples) the ex girlfriend called her friend. It felt like she wanted to know what was happening/spy on us. It makes me feel so uncomfortable. My question is why are they behaving this way and not being welcoming like they are with her new bf? Why do they act like I’m the enemy when I just want everyone to get along?