Andrew R Stewart 

Binder bolts can break from a few reasons, IME the most common is because the bolt is no longer able to remain straight as the enough tightness level is reached. In an ideal world the faces/seats of the clamp/lug/boss will remain parallel, side to side. So as the bolt draws together the two sides of the clamp/lug ears/binder barrel the bolt will see no bending. The reality is that this nearly never happens. Very few clamps/lug ears/binder barrels are preset non straight when in a relaxed state so that after tightening the bolt and securing the post the bolt is held straight. The more layers (separate clamps) or the more slop between the post and tube ID (more room to take up) the greater the amount of likely bolt bending will happen. Add in the greater tolerances of a non round post/ seat tube and clamping system and the amount of slop just builds up. In time the binder slot can too often close up and at this point the bolt head is only contacting the clamp's seat at one point under the head, UNLESS the bolt bends to allow the head to seat fully against the seat. Just like when one bends a paperclip back and forth the bolt will work harden and crack. So a broken bolt generally is showing a deeper issue even if the rider doesn't know it. Andy