What precisely is behind this?

A few - it's impossible to say how many - people in the Labour Party are seriously worried about having a leader who is implacably pro-Palestinian. Their mission is to discredit Corbyn, and it seems their methods are as crude as the hard right media's, whose reliance on claims rather than evidence has been all too prevalent lately. I've just been made aware of a posting on the Jewish Voice for Labour website which with cool, evidential analysis shoots down claims that Corbyn supporters have mounted a massive anti-semitic hate campaign against a Labour MP, Ruth Smeeth who said she had received 25,000 anti-semitic messages on social media. These she claimed came from Corbyn supporters (see Wikipedia: "It’s vile, it’s disgusting and it’s done in the name of the leader of the Labour party, which makes it even worse", Smeeth said during Victoria Derbyshire's television programme). The story made it into the Evening Standard (it would, wouldn't it?). Now, based on the evidence cited in the Jewish Voice for Labour post, I think it would be reasonable for her claims to be properly investigated by the party. If the claims are unfounded, then the next step would be to ask whether she deliberately sought to bring the party into disrepute. 

The post is at  http://www.jewishvoiceforlabour.org.uk/antisemitism/searching-truth-line-abuse-allegations/           

On the same subject, I note that Ken Livingstone has been indefinitely suspended from party membership for the various remarks he has made about Jews and/or Zionism, not least the crass comment that Hitler supported Zionism. Quite right too. In such a contentious area as this, do we really need people in prominent positions cranking up the ante?