What causes "cam walk"?

If he used the stock timing cover, there you go. It distorted the cover and the cam button came apart. You need a billet cover with a roller cam.
At somewhere between $250 and $300?! The idea is to have the area where the cam would push forward reinforced to prevent bending, right? Would it be possible to weld a large washer on the outside of the cover so that, when installed, the washer butts against the water pump to prevent it moving any further. Would that accomplish the same thing?