This has confused me slightly as I thought that when an edit was in review, the post would be locked to further edits to prevent multiple edits being submitted and clashing. Is this not the case or is it only this way for <2k users?
The edit was rejected by the community user. This normally means, a reviewer has chosen to improve on the edit and unchecked the "was this edit helpful" box.
Normally this is used to improve on a bad edit. But if the improver did not change a thing it is a bit suspicious.
Bart's answer within the question comments was correct.
Myself and another user were both editing the same answer at the same time. The post was not locked to edits as we both opened the edit feature before the other user had locked in their edit by submitting it.
As the other user had more than 2000 rep, his edit was immediately approved even though mine was in the suggested edits review queue with two approvals at the time when he submitted his. This in effect made the community user reject my edit.