I believe there's an auto-archiving mechanic on the forum. I'm not entirely sure what would trigger it in these cases (a month seems rather soon, and given the section...), but I'm fairly sure that's where the problem is coming from.

I've very rarely seen this occur, probably because the only old threads I still keep up to tabs with have never given me issues. Said threads are in the testing forum and in a few of the main roleplaying forums.