blakmax, haven't seen a tailboom delam on a 139, so don't know about the failure type there. Have seen and repaired the upper deck delams, and the glue all stayed with the skin on the ones I saw. Basically none left on the honeycomb.
Smells like poor quality control at manufacture to me. I'm sure heat has something to do with it, but if the bond joint was 100% to begin with, I don't think we would be seeing these problems.