Both, some are old school hard left who hate Jews for being successful, some are shall we say individuals with connections to anti jewish groups in the Middle East. Hamas, Hezbollah, Palestinian groups etc. Guess it was logical they ended up allies
Yeah, from my understanding Corbyn has basically supported every terrorist group, well, ever: he supported the IRA in the middle of their bombing campain against England, he's supported Hamas, excetera. Plus the sucessfulness of the Jews generally, which led to the mocking adjustment of their campaign slogan "For the many, not the few", to "for the many, not the jew".
There may have also been an issue of a lot of jews being in the old leadership roles, since they do tend to be more sucessful. Thus, as an insurgent force in labor for a while fighting the Blairite faction, fighting the labor old guard probably involved fighting a lot of jewish labor individuals and groups, just like in the US fighting the Republican "old guard" involves fighting a lot of jews and jewish organizations, probably ben Shapero being the most notable Jewish Republican gatekeeper.
I wonder if there was a similar dynamic in the labor party, where being a successful insurgent force required overpowering Labor jews who used their Jewishness as a shield, which because labor activists wanted to win at any cost, simply convinced them the Jews really were anti revolution and in conspiracy against them. Just as how shapero will sometimes use his jewishness as a bludeon to defend a policy position against criticism can sometimes leave a poor taste in my mouth. Or in Britian that Jewish organization coming out as a major supporter and pusher of Dankcula being fined for having a Nazi pug, or everything the ADL seems to do.
Maybe there really were jewish indeviduals (plus other blairites) who were in conspiracy to them, and they generalized their experiance of labor (opposition) jews to jews in general. With of course the official jewish organizations now coming after them for anti semitism just proving in their minds the Jewish conspiracy against them.
Just to put a potentially more, understanding spin on it.