I guess it's a minor bug/glitch but I've had this on old vanilla playthroughs with no mods.
Confusing and funny and I don't know what causes it.

[img]http://s22.postimage.org/xz5canzbl/Screen_Shot80.jpg[/img]

Probably a yield bug. Or perhaps you dropped an item and he picked it up, but you killed him before he could return it. Making you two permanent friends.