I had a similar experience 6 years ago with a pair of Clark's chukkas. I thought these were a high quality boot and they were in good shape with leather uppers and shallow lug soles. I was about 5 miles back in the woods along a small river when one of the soles separated and fell off while I was walking. I wrapped it with some cordage and headed back to the truck. About a mile before I got back, the other sole came loose and fell off. I walked the rest of the way on the leather that wrapped under, and the insole. The only thing I could think of was that I had gotten gasoline and salt on them while standing in the slush at a gas station that winter, and it corroded the stitches and bonding materials.