I heard pts can get depressed after CABG surgery. Why is that? Asking for a loved one. Thank you.

Mortality Faced. Many studies have shown this to be common as patients undergo an inordinate amount if stress both preoperatively and postoperatively. Being faced with your own mortality and the stress of the procedure has a profound effect on ones psyche. Good post op care by the medical community as well as family support is important. If the depression is becoming a problem medical attention is necessary.
It's true. Depression happens after CABG surgery in 25-40% of people. There's little agreement about "why, " though. There could be subtle brain injury in susceptible people, with reduced cerebral blood flow during the surgery -- perhaps CNS dysfunction after. Also, the surgery & required lifestyle changes afterwards can cause a big disruption in the person's self-view & view of the future. Or others.