When I have encountered anti-Semitism amongst left-wing people it has largely been a product of ignorant people holding a general (and permissible) disapproval of Israel and its policies as regards Palestine in particular, and lurching from that to a wider (and impermissible) opinion that jewish people and the Israeli government are interchangeable and that valid criticism of one is morally equivalent to invalid criticisms of the other.
My own thoughts, for what they are worth, are that Jeremy Corbyn's own personal stance on Israel and Palestine has had two effects: first, he is surrounded by those who do harbour anti-jewish sentiments that are in fact born our of anti Israeli sentiments, and they have moved to positions of power and prominence within the labour party as he has done; secondly, the general anti Israel rhetoric voiced by many who are close to Corbyn and are sympathetic to the palestinian cause emboldens many to give voice to their latent anti-Semitism.
I certainly do not believe that anti Semitism was as prevalent in the labour party until Corbyn became leader, not least because I think Ed Milliband (and Blair and Brown before him) would have had no truck with it.
What appals me is the seeming unwillingness of the current labour leadership to tackle anything as vile as racism - for make no mistake, that is what anti-Semitism is.