Seen this before with the Bender rear mount. When exhaust gets red hot and expands, it flex's downward, bumps in the trail compound this effect. I have seen the Bender Y pipe work its way right out of the slip joint going into the turbo from the expansion and contraction. When the pipe can't slide back into the joint, it has no where else to go but down. The studs hitting the pipe is the secondary effect of the warping pipe. MCX rear mounts are notorious for this also. Added heat and pressure distort the tubes pretty bad, enough to allow them to settle below the protectors and chassis.