From Jim in comments:
Social ostracism. Being ‘Anti-Eu’ when there was zero chance of us being anything other than in it was a position that held no danger for the people who these turncoats socialise with. ‘Oh Matthew, you and your Euroscepticism, you’re so funny, we’ll have to start calling you Nigel! More champers and canapes? Come and meet Sebastian, he’s just started a new Think Tank, its funded by George Soros you know!’
But then suddenly it was all very real, no longer a theoretical joke. A vote that meant something, that meant we could actually be leaving, which absolutely terrified and disturbed the liberal elite. To the extent they’ve gone bonkers over it. And anyone on the other side is a social pariah. A Gammon, a Deplorable, a racist, a bigot. ‘I’m not having people who vote to Leave at MY dinner party!’ So people like Matthew Parris fell straight into line, they didn’t have the nerve to say to stand up for their old views, and rather than find new friends they found new views instead. And like recent converts to anything they become even more fanatical than the long standing adherents.