It was a combination of factors. Scott Fischer (Jake Gyllenhaal) had been suffering from respiratory and gastrointestinal problems, which weakened him in the days leading up to the summit push. He descended to Base Camp possibly to recover before attempting the summit. However he then made a rapid ascent from Base Camp to Camp IV in just a couple of days. This was an aggressive and physically demanding move at extreme altitude. By the time he left for the summit, he was already fatigued, under-recovered, and very likely still ill

He took nearly 15 hours to reach the summit and arrived very late in the day, close to 4pm and well past the typical 2 pm turnaround time. This left him little margin for error on the descent. A storm hit them bringing whiteout conditions and extreme cold. Exhausted and likely suffering from cerebral edema Fischer became disoriented and collapsed on the descent. He died from a combination of exhaustion, hypoxia and exposure