Machover was investigated for antisemitism, but in the course of the investigation it was noticed the leaflet was published by a group ("Labour Party Marxists" (LPM) - a brand used by the Communist Party of Great Britain (Provisional Central Committee), better known for their newspaper the Weekly Worker) deemed incompatible with Labour Party membership.
After four weeks, the party concluded that he did not support these organisations in a way that would lead to automatic suspension:
The Party remains of the view that any reasonable person looking at the evidence available in public (which includes at least one video of you speaking at an event sponsored by CPGB and LPM, 44 articles published with your permission by CPGB’s own publication and primary form of campaigning, the Weekly Worker and 17 videos of you speaking published on CPGB’s website as of 6 October 2017) would conclude that you have given support to at least one, if not both, of these organisations over a period of ten years including while you were a member of the Labour Party. Such support is incompatible with Labour Party membership, so thank you for clarifying that this was not your intention to provide such support.
Below here I have embedded a Storify including my comments at the time of the suspension and links around the reinstatement. First, just briefly, a few thoughts on the case.