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Tester has done a good job explaining it on here in the past. Usually happens when the steerer is cut below the level of the top of the stem - which it would have to be to not bottom out the top cap during preload, the very top of the steerer can be squashed by the clamping of the stem which effectively makes the steerer slightly tapered. Over time the stem would want to walk up the taper.
Can be prevented by leaving the steerer longer, not below the top of the stem and having a spacer between the stem and top cap.
He also used to sell a top cap with an integrated spacer for that. I'm sure he still does.
PhilDAS
Didn't know this, gonna give my top cap a snug up then!