> It's that I'm Jewish, with Israeli family... This was a hard-to-get real world experience in how fucked up people can be for no reason.
Don't be too discouraged. IMHO it's as simple as there being a significant portion of the population who tend to talk shit about other people in their circle when those people aren't around. If asked, they'll often attribute this oddly unmotivated malice to some conveniently proximate reason but, in most cases, if that reason didn't exist they'd still talk some slightly different shit about that same person.
In my experience, these kind of people will, at various times and in various contexts, talk shit about around half the people in their relevant circle. And who's in the half varies over time and each shit-talker can have different individuals in their half. So how does one end up in a given talker's shit-talked half? It can seem almost random but definite contributing factors include the talker perceiving you as better than them in any way (even if you never imply that - and even if it's not remotely correct). It's enough that their insecurity gets triggered even if it's over something 100% imaginary. Heaven help you if you actually are slightly more attractive in some way, have a slightly better job, spouse, education, hobby, hairdo, car - it can be anything or nothing. It's them - not you. And if it wasn't that one thing, it would be about something else.
The truly strange thing is, in my experience, when many of these people shit-talk about their friend group it's unconsciously triggered behavior that relieves some internal psychological stressor. It's almost like some kind of bizarre Tourette syndrome. On another day, in another context, that same shit-talker would tell someone you're their friend, that you're a great person - and, strangely, in that moment they would sincerely mean it. In some ways, I'd almost prefer it if these people were two-faced liars who spend every moment secretly hating me but act nice to my face. While unpleasant, that's at least easy to understand. The reality that they're just socially schizophrenic and almost randomly acting out triggered emotional stress but without harboring any deep rooted animosity toward me is much harder to mentally model.
Once I gained an understanding of this. I learned to avoid not only the shit-talkers, but the people close to them who don't shit talk but listen to their shit talk passively. While the shit-talkers are flawed, insecure people, the regular shit-listeners are just weak and unprincipled. I decided I don't have time to waste on either type. It's also a good reminder to myself to avoid ever slipping into passive shit-listening. Whenever I'd hear shit-talk about someone else, I'd usually politely question the shit-talker on their inconsistent behavior. This pretty quickly ensures no one shit-talks about anyone when I'm around - and it often leads to being excluded from the group entirely. Which I consider an excellent outcome.
Note: Based on the broad circumstances you related, I'll also add a general reminder to always consider the motivations of whoever told you about the shit-talking. Obviously, that's an all-to-common way to stir up drama and/or deepen their relationship with you. Always remember, if they weren't considered a 'safe' shit-listener by the shit-talker, they wouldn't have heard the shit-talk about you. And, of course, exaggerating (or entirely fabricating) the supposed shit-talk they reported to you is another level of shit-stirring.