Event log says that all those thralls were killed by Rotbranch. However, there’s no Rotbranch near my base, at least that I know of. My only guess is that some player dragged one to my base either to save his own butt or just to amuse his/her twisted sense of humor. But then again, I don’t know of any Rotbranch near my base.
I guess that settles it.

Thanks @Birder and @jmk1999 for your help