Just a guess, as I really can't remember how the one example I ever fired worked, but if the extractor was actuated by a sort of camming action near the end of the barrel group's arc of travel (a la S&W No. 3, Webley et al) then that could well explain why so many well-used examples of the little derringer show cracking and/or breakage at the hinge.
If the barrel assembly had to be moved rather forcefully to ensure reliable extraction, then it seems logical to me that this weak point would be where the stresses would concentrate. If the frame was made of relatively 'soft' malleable iron instead of steel, then hysteresis could occur more readily and lead to failure along the stress lines.
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