You may be confusing people DISMISSED from residency with people who leave by their own accord for another field.
Getting dismissed/fired is exceedingly rare, unless you do something really terrible like endanger a patient.
But lost of people realize the field wasn’t right for them to begin with, or when truly faced with the crippling hours and duration of certain surgical fields decide to leave.
I have a good friend who quit gen surg residency for PMR. His wife basically said she was going to leave him because she couldn’t continue raising their 2 small children alone with him never there. He would’ve made a phenomenal surgeon, but at age 30 when you’ve got a family sometimes those things take precedence