Well, my favorite personal "but I'm too lazy to test it or even think too hard about it" hypothesis is that it is likely often a long term consequence of heat damage to the base from hot waxing too hot - and perhaps damaging the polymer chains so they are susceptible to oxidation. Alternatively, maybe some similar heat induced change in some component(s) of some waxes? But that's just a lazy set of guesses - been too busy fighting the rockered/fat ski battle to think to hard on this one . A few years ago when we thrashing around about this, someone with access to a legit lab offered to analyze scrapings of that gray/white stuff. I don't think anyone took him (I think it was a him) up on it. And I'm definitely too lazy to search for that post.
In an experience semi-similar to osloskier's - a few years ago I & the rest of the family drifted away from hot waxing and started just doing a really firm cork-in of a thin layer of Hertel wax. Or, in one case, just not bothering. I have not even plugged an iron in for 2+ seasons. To the best of my recollection, none of of the skis acquired in the household in that window (aka those never hot waxed) have ever shown any sign of that magically appearing layer.