People don't have perfect reflexes, physical coordination, and situational awareness. Someone could be checking their blind spot at the exact moment the person

People don't have perfect reflexes, physical coordination, and situational awareness. Someone could be checking their blind spot at the exact moment the person in front of them slams on their brakes. Someone could move their foot over from the gas and for no particular reason completely miss the brake pedal, or sneeze at the exact wrong time, or be stung by a hornet they didn't know was in the car, or simply not react fast enough. Humans are fallible and make unintentional mistakes. It's surprising that humans are as good at driving as we are and that most of us aren't making catastrophic mistakes on a regular basis.