I'm not a moderator but I have noticed the same issue, ever since the last software upgrade. There is a default that can be set by the admin on how long to allow someone to not post before they are automatically logged off, so I assumed they adjusted this to get the many people who are logged on but not visiting off the pipeline to speed things up. It also probably is how they are able to show us who is on and off line in their profile box next to their posts, which is a useful feature at times.

Since you can still read everything and only need to log on to post, I haven't found it to be a big concern. I just stay logged off until I have something to type...