Dam-breach experiment: outburst flood from an overtopping lake (USGS)

Why do natural and man-made dams quickly collapse when the water level exceeds the height of the barrier, rather than just allowing the water to gradually overflow?

In this dam-breach experiment, a 1-m deep overtopping pond triggers a small flood by gradually eroding the compacted-sand barrier. By USGS, Oregon (2012-06-13).The lake behind the barrier is 23.7 m2 in area (not 10 as wrongly printed in the video).