People with monster quads like hoy.

Actually, the risk of unclipping from actually pulling out of the pedal (not from bad twisty form) happens during standing starts and jumps (massive accelerations before a flying 200 or during a match sprint) when guys put out at least 1,500W. Of course world class men can output 2,000 - 2,5000+ watts. The women can crank out over 1,800, which is why they also use them for standing start events, 200s and match sprints.

This event was a scratch race, which has a rolling start (think NASCAR) and the accelerations are relatively subtle like a road race.

Also, as with all track racing, it's has to with power/weight ratio. Shelly Evans is probably 125lbs soaking wet. So, she doesn't produce as many watts as some of the sprinters, but she weighs much less, too. But, she is still relatively easy on the pedals.