Very simple...the 1/4" TE stock simply wasn't enough surface for the hinges to bond to effectively with epoxy alone. If I'd used some of the expanding glue, put in hard points behind the hinges, used thicker TE stock...any of those would have prevented the problem. For that matter, some of the heavy duty CA hinges probably would have been ok.

Just one of those "shoulda trusted my instincts" moments.