I knew and talked with several of the folks that were there on that day. The fill was complete and the thick sheet piling had all ready been driven. They got in a rush toward the last part of the fill, which was the east side. The fill hadn't enough time to properly settle and some say that the sheet pile had cut through a small seam of a slippery type of mud. That cut off support for the fill above and the whole section slid into the pool. Resulting wave raced toward the hills on the south of the pool and then rushed back toward the fill. Nearly slopped over but the weakened fill held, plans were revised and the upstream base of the fill was widened and the project delayed about a year, but the dam emerged stronger than ever.

It's interesting that witnesses report the whole fill shaking violently before the slump finally occured. Perfect conditions to create the liquefaction effect mentioned but none of that happened because clay doesn't liquefy.  

Rafe Sigmunstad has put together a nice website on the dam and it's construction. He covers the slide and the boom towns, among the other aspects of the construction. Well worth reading for those interested. http://www.fortpeckdam.com/