A post-mortem is usually one analyst reasoning on the group's behalf, and it usually names the most visible failure: the outage, the returned part, the picking error. Those are consequences. The causes sit further back, spread across the people who saw different parts of it.
True Causes turns the review into the group's own analysis. Everyone proposes causes anonymously, so nobody writes around a manager. The group then answers one question at a time, does this cause significantly influence that one, and a link is kept only when three quarters agree. A split is never quietly dropped: it goes to another group, and if that splits too, it is published as contested.
Several teams or sites can run the same question separately and merge their maps, which shows which causes they found independently. A private room can also take in what the public rooms on its question found, with nothing of its own going out.