Basically there's a coup started by a PR company called Portland House - one of the Directors is Alistair Campbell, it's linked to Tony Blair, has a ex-Sky man as another Director, they organised this coup 6 months ago regardless of the referendum result. They say that it is Corbyn's fault that only 63% of the Labour party voted to remain (1% less than the strongly Remain SNP party), but what it is really about is an orchestrated attempt to force him to resign before the Chilcott Report is released next week in which Corbyn has already said he will demand charges for anyone found guilty, including Blair. Blair doesn't want the leader of the Labour Party in the dispatch box demanding his head.
The problem comes that the PLP (Labour Parliamentary Labour Party - essentially the MPs) are so out of touch with the public and with what party members want that they tried putting different shades of Blairites up for nomination - Milliband, Cooper, Kendall, Burnham. The result was the Labour Party hemoragging votes and membership numbers hugely since 1997, to a new low in 2010. Little interest was paid in the new leadership campaign until Corbyn was put forward. When they realised that Corbyn might get on the ticket and how much public appeal he had, the PLP panicked and tried to stop his name being put on the ticket, then they tried changing the rules. After that failed they started a sustained campaign of personal and professional attacks in a smear campaign - totally forgetting about the Tory's - at the same time as Abstaining from voting against the Welfare Bill.
Corbyn won with the biggest landslide of any leader of any party EVER (a quarter of million people voted for him!) he won 60% of the vote in a 4 horse race! In spite of the mud slinging, smear campaigns, the lies. People saw through them and his honesty and the fact he actually had policies that made sense, worked for working class people and answered problems important to them (rather than saying 'I'm a Woman' like Cooper) had huge appeal for disaffected people beaten down by years of ideological austerity.
Now at a time when he has won a party leadership, increased party membership, won bi-elections and did better (or as good as - soz can't be arsed to check) as Milliband at his height in the local elections. But the claim he is unelectable is still made and any of the hundreds of thousands that support him (as opposed to the few thousand that support the other Labour candidates) are dismissed as either far-left 'loonie-lefties' who couldn't possibly represent 'normal' people, or as needing a heart transplant, or even worse as Tory's pretending to be supporters to keep Corbyn in power!
As such they say we need to ditch 'unelectable' hugely popular and democratically elected Corbyn, disregard the will of the party members, believe a 177 MPs and a PR company (funded by HSBC funded) campaign, forget about the 350k plus members he has attracted to the party. Ignore the many warnings from members that they will defect if Corbyn is axed.
And instead place a more right wing member in charge who is more likely to win back some of the 25% of the voters who voted Tory. While at the same time further disaffecting the 35% who didn't vote. Not forgetting the old Charles Kennedy period Lib Dem supporters who defected after Nick Clegg's coalition with the Tory's, or UKIP supporters - who now that they have achieved their aim of getting out of Europe may actually want to achieve some other aims the parties supporters have polled strongly in - namely re-nationalisation, protecting the NHS and other policies that Corbyn has polled strongest among their voters in.