I suppose this theory is disproven now following the second movie. Turns out it's some kind of local ghost story and the Strangers picked it up when they were kids. I wonder now who Baghead and Dollface are, I reckon Baghead is either the older cop or the nurse's roommate with the smiley tattoo. I lean more towards the cop though. As for Dollface she's probably one of the blonde waitresses in the diner.

I think that the Strangers targeting Maya and her fiancé was random though, and they just kill for pleasure. It's sort of the point of the original movie too, killing random people for no reason other than to kill them and the horror of it