David Eagleman's perception speech discusses the reasons for experiencing events in slow motion when put in life-threatening situations. Eagleman claims that while time seems to move more slowly, this is actually a trick of the brain.
After doing a series of time perception tests, some involving bungee jumping, Eagleman came to the conclusion that people don't experience these stressful events in slow motion. However, they remember them in slow motion. While under distress, the human brain burns a lot of energy and takes in a lot of information. This provides people with a detailed account of the traumatic event. Because people remember these things in such great detail, they naturally assume that these events lasted longer than they actually did.