It sounds like a murder mystery. Everyone had a reason for promoting the idea that the Labour leadership ballot was being undermined by Tory infiltrators and ‘entryists’. Anything that could destabilise the ballot and make it look like a mess is good news for the right and much of the press. It suits some groups on the hard left to seem bigger than they are. Once it became clear that Corbyn might win, anything that de-legitimised his victory was music to the ears of the Labour Right. And some knew, deep down, that if enough of a storm was created about infiltration, this would provide cover for the party apparatus to deny more leftwing activists a vote, and that this could, just about, influence a close result.

Now, after a lot of noise about weeding out infiltrators, and one batch of more obvious candidates for expulsion which included Ken Loach and other prominent figures from other parties, there is a clear drip-drip of new rejections. Many are young left wing activists from the student movement and other social movements who joined Labour, enthused by the Corbyn campaign, and had every intention of remaining in the Party. The reasoning behind these new rejections looks, at least at first sight, murky.

Hattie Craig is a recent graduate from the University of Birmingham, and a relatively prominent activist the National Campaign Against Fees and Cuts – the biggest organisation on the student left. “I was inspired by the Jeremy Corbyn campaign,” she says “and the possibility that Labour could truly represent and fight for those most oppressed in society.” Like many others, she has received an email stating that she was rejected because “we have reason to believe that you do not support the aims and values of the Labour Party or you are a supporter of an organisation opposed to the Labour Party.” But, Craig tells me, she has never been a member of any other electoral project – or indeed any other party at all.

A large number of the rejections appear to be students. Rachel O’Brien is also a student activist and a current student in Birmingham. She describes herself as “heavily critical of the Labour Party and their current policies – but not opposed to the party as a whole”, and has, like Craig, never been a member of another party. “I think they are missing a nuance there.”

Marienna Pope-Wiedemann, another rejectee, is a freelance writer and producer who was also politically active as a student and has continued activities outside Labour. But she is no longer in anything else and, as she points out, being active outside Labour is rather inevitable: “most people are active through organisations other than Labour because the Labour Party has been so long disconnected from community struggle and afraid of taking on the big issues,” she says. “This is the first time my generation has seen Labour stand up and fight for ideals most of them are too afraid even to speak of anymore.”

Bernard Goyder’s example looks even stranger. Now a financial journalist, Goyder was a student activist in 2010, and then involved in Occupy and a number of housing campaigns. “I was involved in Young Labour as a sixth former, and joined properly in 2010, days after the election. I rang the party to notify them of my new address, and found that it had lapsed a few years ago.” Now his application to re-join has been rejected.

In the past, Goyder voted for a mix of parties – including the Lib Dems and the Greens – but he campaigned and voted for Labour in 2015. “In 2010, the NUS and the parliamentary system failed young people so we had to make our own politics. I’m proud to have been involved in the 2010 student movement and the 2011 occupy protests, but see no contradiction between this activity and ‘the aims and values of the Labour party’”.

Quite apart from how this all looks, Goyder’s cases raise another rather glaring issue: people changing their minds. Labour at high school, fighting on the outside at uni, and then back into Labour – it’s a well-trodden path. By definition, everyone now joining Labour is, to an extent, changing their mind; many are being swayed by the real possibility of an anti-austerity alternative in the form of Corbyn. Across the course of the campaign, many thousands have changed their minds about Labour – giving Labour hordes of new members and supporters, and a good deal more credibility in places where it previously had none. This was, after all, the entire point of having a supporter sign-up system – and many will have joined, as the system’s architects hoped, from Labour’s right as well as its left.

Large swathes of the PLP and Labour establishment were once in, or voted for, other parties. John Reid and Peter Mandelson were once members of the Young Communist League, and Shaun Woodward, current Shadow Cabinet member, was a Tory MP until he crossed the floor of the House in 1999. Hell, some Labour MPs have campaigned for other candidates while in office: when Lol Duffy, a member of the now-proscribed Socialist Organiser platform, won the Labour selection in Wallesey in 1987, neighbouring MP Frank Field openly refused to support Duffy as the Labour candidate.

But for a large chunk of those who have had their membership or supporter applications rejected, it doesn’t even get that far. For Craig and O’Brien, who have never been supporters or members of other parties, the implications of being rejected seem clear: “I had already voted when I got the email, and it is also very clear from my Facebook that I support Corbyn,” says Craig. “I do not think this is unconnected.”

Of course many, if not all, of those who have been rejected have been critical of Labour policy and the Labour leadership – often in the public domain and on social media, where Labour staff are reportedly trawling for evidence. But then, within Labour’s broad church, so have most members. In fact, so has every candidate for leadership. Those of us on Labour’s left flank, many of whom have door-stepped for the party, held minor office and never voted for anyone else, could be forgiven for nervously refreshing their email inboxes.

None of this is helped by the vigilante attitude that seems to have gripped some on the Labour Right. One post currently doing the rounds on Facebook states: “If you know that someone who has recently signed up as a member, supporter or affiliate, who is not in fact a supporter of the Labour party, you should email their name to leadership2015@labour.org.uk with proof.” The post concludes: “Please do report anyone you suspect should be ineligible – and you too could be called a star by the Compliance Unit”.

There is no way to verify whether or not Labour’s Compliance Unit have in fact called informants ‘stars’, but adverts to report your neighbour like this one, posted in relation to a university labour club, are breeding an atmosphere of McCarthyite fervour.

If, as the post says, “any written expression of support for a party or group other than Labour, or opposed to Labour” is enough proof to have you expelled, then we had all better be careful about praising so much as the individual policies of another party. That could go for Liz Kendall’s supporters just as much as Jeremy Corbyn’s