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From quoted page said:
The bittering agent generated from hops while boiling beer wort is a compound called isohumulone. Ultraviolet light can degrade isohumulone all by itself. But it turns out that visible light can also induce isohumulone degradation -- it just needs a helper molecule, in the form of riboflavin. Once the proverbial ball starts rolling (or, in this case, the electrons start hopping), a series of reactions take place that eventually produce the compound 3-methylbut-2-ene-1-thiol. That mouthful, known colloquially as 3-MBT, is your skunk. In fact, 3-MBT is chemically very similar to one of the three main compounds found in a skunk's defense spray.
The amount of riboflavin in malt is minimal, though it is present. It's mainly produced during yeast growth stages, if memory serves. Once that's in the wort in sufficient quantity, you run the risk of the reaction described above.
It seems like a decent enough source to me. Let me know if you want me to find something more authoritative, or something that speaks to riboflavin levels at different points during, or before, fermentation.
