You fall at work. You're on the top of a ladder when someone bumps it and you lose your balance. You don't fall far, but you hit your head on the way down. Everything goes dark for a moment, and then you wake up to find people looking down at you.
Much of the time, drowsy driving happens because a driver simply did not get enough sleep before they got behind the wheel. Maybe it's a college student who stayed up until 2:00 a.m. the night before an 8:00 a.m. class. Maybe it's a truck driver who had to get up early to pick up a shipment on time. It happens in a lot of different ways.