I've experienced something similar, and it was the weirdest experience ever. I've never before had any sort of social problems, even at school I was never involved in any sort of girls-leaving-each-other-out nonsense etc.
I had a close group of 3 girlfriends at work. We were all expats together so much tighter than 'work friends' usually are. I was the least 'in' with the group as I had young children and they didn't, but they always involved me in things and were fantastic with my kids. One even stayed overnight with me in hospital when I had a baby. All 3 took it upon themselves to arrange for my husband and I to have a night away while they babysat THREE kids including a new baby. Holidays together. Birthday surprises. That level of friend!
Then suddenly... They just stopped including me. There was a group holiday arranged, with these women and some other colleagues. I arranged to stay in the hotel next door as it was more kid friendly. I knew things were off before we went but I was also experiencing the breakdown of my marriage by this point, so I just couldn't give it headspace. Only one of them even contacted me during the holiday and I didn't meet up with the group.
Back at work, where I was technically the line manager of 2 of these women, one was ostentatiously professional and courteous and the other wouldn't even look me in the face. It was ridiculous. I was months from leaving the job and having an awful time at home, so I didn't even ask them what the problem was. They had cut a woman out of their friendship group before I was very tight with them, and in hindsight I should have seen that behaviour for what it was. I'm ashamed now that when that happened, although I did meet this woman and say I had no idea what was going on (I didn't, really) and that I had no problem with her at all, I didn't push them to consider their cruelty in just cutting her off. They had previously holidayed together as a trio several times a year for two years.
Anyway, I remained friends with one of them. We have all since moved on to other jobs/countries. According to the one I'm still friends with, they ask after me and like to hear how I'm doing. WTF?! They cut me off when I was at the lowest point of my life. With no explanation at all.
I genuinely know I did nothing wrong. If I had upset them in any way and they'd spoken to me, we could have worked things out. I really valued their friendship. It doesn't bother me anymore but there was a time when I was very confused and upset by it, and it did affect my confidence trying to make friends in my next job.
After I moved job and we were all still local, I met up with another mutual friend who had been in our wider group. I didn't want to shit stir but I told her to just be mindful, and make sure she maintained her friends from outside work. That was in the September. By Christmas they had cut her off too. They lied to another friend, saying that she never replied to their msgs etc. I'm reality, when they were stuck in isolation after re-entering the country, she was offering to send them snacks etc.
All just very ridiculous. I can only assume that they haven't matured past about 15 years old, and that they like their friendship circle to function a very specific way, with everyone in their place, and when you don't they just cut you out. The ringleader has no friends from school or uni and says she was bullied. I now wonder if this is true (in which case, maybe she now treats people badly as a result of this? I feel like I'm reaching with that idea, though!)
Anyway OP, just to say solidarity to you! Glad you've moved on. I honestly never thought I'd experience something like this; I'm one of those people who is friendly with everyone and has never struggled with close friendships. I can only assume these women have their own issues and are probably deeply unhappy.