Difficult to tell without seeing them. What kind of shafts were they? Could be short shafted, meaning that the shafts did not get all the way to the bottom of the hosel, which puts mores stress on them. Could be they were over prepped (sanded) resulting in thinner walls, which can lead to breakage. If the shafts were graphite and too much epoxy was used, inside the shaft the epoxy can cure above the top of the hosel, which creates a shear point. Not as much of a problem if the shafts are steel, but you still don’t want to use so much epoxy the inside of the shaft is filled above the top of the hosel. Those are just educated guesses. Again without seeing them, I couldn’t give you a definite diagnosis.