It;s not the surgery. It's a "hard-wired" lizardian response in your DNA. When the body is threatened (low immune system or serious injury) the "lizard brain" kicks in and the person becomes angry and often verbally abusive to others. Back in the days of sabre tooth tigers (I know we didn't exist at the same time period, but you get my point) an injured animal would become nasty to keep predators away. This can be seen in patients who experience blood loss at auto accidents. They throw out four letter words until a blood transfusion brings their hematocrit to normal and like magic - a switch turns off and they become normal and pleasant.