Yep, I noticed that too. We do have a fair number of inactive admins for this channel, its possible that someone misclicked and re-pinned that? We've also seen weirdness like the GWT 2.10 release announcement and the first 3 or so replied deleted for no reason, so perhaps something odd is happening with Google Groups itself...
I'm not even sure I had access to pin/unpin messages until recently, but I seem to now. I'll update to get 2.12.1 as the latest pinned message.