If there are regular slaughters, smart animals might notice a pattern. "We never saw Fred again after he went to that strange shed. Then we never saw Tom after he went there. Why the hell are they dragging me over there now?!??"

This doesn't mean that any pig could articulate "I'm going to die". That isn't necessary. Nearly every ungulate (maybe not moose?) has a built-in mortal fear ready to surface at any time. New events, or repeated events with bad associations, bring out that fear. It takes a great deal of training to relax that.

The whole concept of thinking about animals dumber than they are makes killing them more acceptable.

It's reading emotions/social cues, not objects. Animals are frighteningly good at that.

Maybe dogs. Not pigs. I've seen it happen several times. The pigs didn't put up a fight or act at all abnormal.