The quick reaction team had not yet verified the location of Chapman's original team that had slid down the mountain. One of the most plausible hypotheses as to how he was left behind is that, once they began to retreat down the slope, they passed over Robert's dead body (seen clearly in the video), and mistook it for Chapman, who was inside the bunker. However, there were multiple contradicting stories by eyewitnesses (even by the same people contradicting themselves) in the intervening years between 2002 and 2018.
I won't spoil the read for you - but suffice to say that the book goes in to great detail to put together what actually happened, and reports of a deliberate effort of some officers to bury the truth. They all get named in the book. Very sad, really.
The terrain details are much more apparent in the video after reading the book. The reason the team lead seems to linger before going up the mountain with Chapman is that he stumbled getting out the helo - falling through the thigh deep snow, and face planted with his NVG's on. As soon as he cleared them out and reoriented, he took off after Chapman.
https://www.amazon.com/Alone-Dawn-Re.../dp/1538729660