it could be fatigue related...but its hard to tell from the photos.....he has ripped off two of the prongs, which would have taken two seperate cracks..maybe at the corners (tis kinda symmetrical..) wally (mr rimjam) said that his pads made this happen a few times, because the (think it was a zinc alloy) he was using was quite soft, and after a while the backings had deformed enough to give a substantial "wobble" to the pad. Then when the brake was applied, the backings are actually hitting the guide prongs- nice brittle casting + impact...snap. Not a problem on the heatsinks though, because they are made from a harder material that doesn't deform to the same extent during use... clearly something has gone wrong though...! did you have the pads quite far away from the rim? adam