This is something that has happened twice in the last week. I've had ongoing stories that I had previously bookmarked but edited the bookmark to remove them from the Active Serials Queue (leaving them in the Reading Queue) get re-added to the Active Serials Queue with no interaction on my part. The first time it happened, it seemed to be all the stories that had unread chapters. This time it seems to be all the stories that don't have unread chapters. I'm not sure what is happening, but I'm curious as to whether this is something that is going to continue happening. If it, I'll probably adjust how I use the queues rather than have to go edit those bookmarks individually to put them back the way I like them.
ETA: I just started going through and removing them from my Active Serials list and found that quite a few of the stories are ones that have been inactive for quite a while and I had removed them from both Active Series as well as my Reading Queue prior to today. I think every story that wasn't marked as complete was re-added to my Active Serials list (including the ones I'd previously removed).
ETA: I was just reading the post about somebody who lost their library and saw the post about somebody who had their series list messed up. I just checked mine and it is really messed up. There are series and universes that I've never heard of, let alone added. There are are also some series and universes missing. I can't remember what exactly was there well enough to tell exactly what is missing, but I know some are missing. And there are quite a few extras.