What was Krakauer most worried about as he began his descent?
First, in any big mountain climb the descent is always the most dangerous part. Not only are climbers tired, but down-climbing can be difficult because one does not have good visibility of one's route. Fatigue increases in Himalayan climbing exponentially with time spent in the "death zone" over 25,000 feet, where the air is so thin that the human body cannot acclimatize.
In this specific climb, there were several factors that exacerbated the difficulty of the descent. The first and biggest was that there was an approaching storm, which brings the dangers of cold, icy conditions and white-outs making route finding impossible. Next was the time. Several people were slow to summit, meaning that they started the descent late, leaving insufficient time to make it to camp before the light failed.