It’s a new feature of shadow lands we get to try out early. The ghosts of whom you vanquished are aggroing you but you can’t see them.
I usually get this message cause my minions like attacking random things for the fun of doing so, or that bunny rabbit I aggro’d randomly but somehow didn’t one shot it so it sits there shouting obscenities at me in a language that I cannot understand.
As a stealthie, I agree. Very annoying.