*JUST my opinion, not stating anything out-right, no definite facts. Just observations from my own experience*
Thought of this today when I was feeding the snakes at the pet store I work at, specifically when I was feeding our male pastel we have.(who is in an enclosure by himself).
I was feeding our pastel a f/t rat pup today, in his enclosure, when my boss came around the corner and instantly started yelling at me for feeding the snake in its cage. Stating "you know if you feed a ball python in its cage it will make it ALOT meaner". WHAT I THINK, is that people mistake a good feeding response for aggression. All of my snakes eat in their tubs, which I do believe makes them feel more comfortable, and all are eating machines. Now I just opened the tub to my little butter girl, and when I went to move her water bowl I saw her head poking out of her hide and following my hand everywhere it went. It did look like she was getting ready to strike at me, so i placed my hand near her to see if that's what she was going to do, and nope, no bite. My dumerils will do the same thing. They may get back in the "S" shape looking like they're going to bite (which can be very intimidating to a novice keeper, and can be easily mistaken for aggression) but I think they just LOVE TO EAT.And I think when they rear back I think they're just waiting for a rat to pass in front of them.
Just my thoughts on what I noticed tonight while feeding.