This has been happening for a while now, since about the last update you guys made to the site and uploads. I've just been observing it for a while, waiting for a good chance to show it happening.
It's not always anime related. I've had wallpapers of real life subjects show up with tags that I'm not subscribed to. I'm not subscribed to their uploaders either. I've checked all the tags on the wallpapers that show up like this and have confirmed that I wasn't subscribed to them.
The only thing that might shed light on this is the fact that I used to be subscribed to anime girls but I unsubscribed back in January, I think. That doesn't explain the real life wallpapers that show up though, although I can't recall what tags were common to them and whether or not I used to be subscribed to them as well.
I remember the first time this happened, the item was so out of place I immediately said "WTF? Where'd this come from?" and started investigating.
EDIT: After refreshing the new subs page, I manage to rearrange the thumbs and get these four to show up:
