Al from ASB let me in on what it is, saying that it's caused by improper force applied to the inner/outer bearing races. It's also important to insert (on the rear wheel) the bearing on the brake rotor side, first. Then place the spacer and the other bearing, but keep constant pressure on the inner and outer bearing race, until the inner race comes in contact with the spacer. (Be sure not to mushroom the spacer as it is a softer aluminum.) After that, be sure not to snug the axle down too tight and crimp all of the assembly.