I have been a volunteer firefighter for many years and have been in that situation once. It is visceral, a gut reaction. You have no control over it.
When we first saw the reports of the planes hitting the tower...phone service..both landline and cell, was intermittent because of the heavy volume. I'd been trying for an hour to reach her..but no luck. About 30 minutes after the first plane hit...they started to evacuate her building...she was on the 23rd floor..they walked down...she was about halfway down when they cancelled it, and told them all to go back to their offices...as she was starting back up, I got through to her on her cell...I told her to leave, do not go back.. go down, get out, and start walking north..we later met near Canal Street..I had to walk most of the way down from from Grand Central. As she exited her building, and started to head across the plaza..some of the bodies were landing. Several were quite close to her and others, before they started roping the whole area off and keeping people away as they left the victimns..
The past decade has bee difficult for her..she got help to deal withe the trauma...is still on some meds, and has tuned down a few jobs that would have had her working in skyscrapers...