Greg,
I'm not a contractor and my only experience with dams dates back to my kiddy years trying to plug up small streams (with limited success) so this is only an opinion.
Your first photo may give some leads to the resultant dam failure. To me, the material used to build the dam does not look like "clean" fill. In the photo it looks like there are alot of tree roots and sticks mixed in which could provide an avenue for water to seep into the dam (you mentioned there was a large tree there). Once water gets its foot in the door, trouble will likely follow. In addition, I wonder if the slope on the backside of the dam was cleared of vegetation allowing the fill material to bond correctly. This follows the thinking stated in JBL's post.
Again, this is just my opinion. I have no right to question a cat skinner with 20+ years while my resume only reads as an armchair operator. Good luck with the repairs and keep us posted.
