From Corbyn's biographer, Jalima.
"I think what’s changed is he has so many issues on which he’s picking a fight with his fellow MPs, he’s so implacably opposed to things which his PLP (parliamentary Labour party) colleagues hold dear, that this isn’t one he feels strongly enough about to pick yet another fight over.
I think he’s seen, too, that the EU has potential to help in the causes he believes in. So in an interview over the weekend he was talking about using the EU to campaign for workers’ rights, worker solidarity.
I think he has moved on slightly from feeling that it was a trading-bloc which was hostile to workers, to seeing it as potentially something that would promote the values he believes in.
Maybe it’s that old saying of Napoleon: “Never interrupt your enemy when he is in the process of making a mistake”? Because the Tories have a majority and are in power for five years, but they could tear themselves apart, potentially, over Europe.
You are absolutely right. Europe has always been a bogey issue for the Conservative party, and I’m sure that any savvy leader of the Labour party thinks ‘why should I get intrude on private grief?’. So it’s much wiser for Corbyn to get out of the way, and let the Conservatives get on with fighting each other.
And how many Labour MPs, and Labour voters, when the referendum comes, might actually vote ‘out’?"
He sees the EU as better for workers rights than the UK.