If people start arguing in the thread and there are too many posts to delete to bring the thread back on topic
Against guidelines, or the thread topic may be within guidelines but several posts within are against them and it will take too much time to clean up the thread
Poster asks questions and the thread turns to attacking the poster (or a poster in thread) rather than addressing the subject/topic itself
Generally disruptive to the boards - some of us have been here for a long time and experience has shown us that even when we give benefit of doubt certain threads will turn contentious, so we lock them before they have a chance to turn ugly
A topic was already posted and turned ugly so thread was deleted or locked and someone reposts a new thread about the same subject
Since your thread was in the rent/trade board, the Required Reading indicates that all postings will expire 1 month after approval. This is the likely reason your thread was locked.