Final answer:
Krakauer cites unstable weather conditions as the reason they might not be able to make a summit push at the scheduled time, which aligns with the challenges faced in mountaineering such as extreme cold and low oxygen levels.
Step-by-step explanation:
Krakauer gives the reason for believing that they might not be able to make a summit push at the scheduled time due to unstable weather conditions. This is a common challenge faced by mountaineers, as depicted in the given reference which shows that climbers on Mount Everest must accommodate extreme cold, low oxygen levels, and low barometric pressure in an environment that is hostile to human life. These harsh conditions, especially the unpredictable weather, can greatly impact a climbing team's ability to stay on schedule for a summit attempt.