The aluminum work hardens from the vibration resulting in cracks or breaks in the aluminum.
Thats one of the reasons why you need to adapt the air pressure on the tires to accomodate for the stiff mags versus regular spoke rims...as a for instance, my Kevlar tires say to inflate from 40 to 60 PSI...Bike weighs 80 pounds, i weigh 217 pounds...i set my tire pressure at 35 front, 38 rear...bike rides perfectly and tires do not look flat and handling is just fine....This inflation method makes certain that you have a little forgiving "give" to the tire and rim combination to avoid any jarring vibrations that can eventually cause stress cracking and also insures a comfortable ride as well...DAMIEN
Ps...Also allows for naturally occuring "heat" expansion of the PSI in your tires at the higher speeds we go for as well.