This happens a lot on Boards I find, and it is incredibly annoying, and tbh if I wasn't a member, it would put me off signing up.
Dav mentioned a few Months back that sign ups are down, and is it really any wonder when this ****e happens.
For example
this thread in Accommodation and Property has 153 posts in the last week, and it was just locked, because the OP didn't reply.
This thread in Bargain Alerts was closed back in July for no reason and with no explanation.
Locking threads is a really annoying issue with boards. For boards to keep on growing this has to stop.