Bill Hempel posted on this earlier this morning before the thread got deleted. It was NOT a structural failure.
He explained he had switched flight boxes. All of his regular screws, etc were left at home. He improvised using the wrong screws and he remembers, he thinks he only used one of the retaining screws instead of two. It had nothing to do with the structural integrity of the plane. He said it slid out on the tube, then broke.