Corbyn, who led Labour between 2015 and 2019, was initially expelled from the party and had the whip withdrawn in April 2020, under the leadership of his erstwhile ally Keir Starmer.
The expulsion came after Corbyn said that claims of antisemitism during his time as a leader had been “overstated” for political reasons, in response to an investigation into his handling of the issue.
I guess he could be trying to intentionally play a spoiler role. Like, he’d have to want Labour policy changes, believe that Labour adopting those changes wouldn’t make Labour lose in the general election, and believe that he could split off enough votes to cause Labour to lose so that it has to give him those concessions if it wants him not to run.