This seems to show that it was the "spillway plug" that eroded and failed. I don't know what a spillway plug is, but the narration seems to imply that it is designed to fail in emergency situations.

In the beginning of the video, the narration talks about the spillway plug being a feature designed to erode in a fashion that delayed/slowed the inevitable failure of the entire dam. In this case, the spillway plug helped the dam release water in a semi-controlled fashion over a series of hours, instead of an uncontrolled fashion in a matter of minutes.

If a embankment dam is overtopped, it is going to fail. They designed it with a slightly lower section on one end so they could be sure if it failed, it failed in a predictable way.

Interesting footage. There's a good before/after comparison at the end:

https://youtu.be/tDmwo5nsWfQ?t=9m4s

Woah. Could you give me some more VCR, please?

This was the coffer dam for the construction of Auburn Dam, work on which was halted, but the coffer dam never removed?

Excellent video!