Sometimes it gets closed when i do a quick reply and sometimes it doesn't. Very strange.
Depending on what keys you hit when replying (as a Moderator), the "Close this Thread" box gets checked under the "Quick Reply" box. The post gets posted properly, but immediately the thread gets closed. Pretty effective tool for Mods getting the last word. I don't know the cause of this, but it just happened to me in a different thread (once again).
As long as the "Close this thread" box is unchecked, no problems.