Originally Posted by jimmuller

You're right that the spindle doesn't touch the cup. But the bearings "going the opposite direction" isn't why the cup can unscrew. In fact, there is little or no friction between spindle, bearings, and cup. The bearings do rotate the opposite direction from the spindle but that is because the are rolling around the inside of the cup in the same direction as the spindle turns. Viewed from the drive side, the spindle turns clockwise and balls roll in a clockwise circle.

The reason the cup can be unscrewed is because it is an inside circle. The image shown here illustrates the behavior. If there is any deformation in the shell it will elongate the shell to make it slightly bigger than the cup which has no reason to elongate. Even if stress does not deform the shell, the threads of the cup are necessarily slightly smaller than those of the shell. The cup then rolls around the inside of the shell. Since it is a smaller circle effectively than the shell it takes more than one revolution to roll one loop around the shell. By the time it gets back around to the starting point it will have turned some small amount.

It is indeed a real problem. The solution is to make sure the fixed cup is very tightly torqued. Use of a locking material such as Locktight might be useful too. "They" say you should never removed the fixed cup, simply because getting it back tight enough isn't easy.