This is a not uncommon phenomenon. Jews tend to be among the most ardent of lefists; then, when they detect a conflict between the ideas of the Left and their own ethnic interests, they transition to the Right. This is exactly what lay behind the emergence of the neo-conservative movement in the US. Jews have mobilised big-time against Corbyn because he embraces the "Palestinians", has "dialogue" with Iran, and refuses to anathematise Putin. Almost every Jewish journalist, including far-lefties who could normally be expected to have embraced Corbyn's ideas, has come out strongly against him, always citing some abstract moral justification for their opposition rather than admitting that their ethnic interests are at play.