I think these weren't bonded/vulcanized adequately. Most of my experience with tire wear is the same as yours, normally the tread rubber stays attached to the casing until it is worn completely through and the casing shows.

However, I recently had two Kenda 700-23 "Konstrictor" tires fail the way yours did, the tread rubber separated from the casing before it was worn completly through. In fact, one was a front tire and showed no tread wear (front tires usually don't) when it failed. I e-mailed Kenda about this but never got a reply.