I have noticed that many of the bows on the auction house have random Witch Doctor skill buffs. I have noticed the same phenomenon with Demon Hunter skills on chest armors and Monk skills on helms. I know that there are rules that govern how the server picks affixes to put on an item and I was wondering if anyone could shed some insight on what causes this to happen. Thanks

The client-side Affix data only tells us which Affixes can spawn on which item types, and what the corresponding alvl (affix level) required for it to spawn.

Unfortunately, what we don't know is whether all Affixes have the same probability of rolling as any other Affix. In D2, those probabilities (iirc) were not all the same (though most were identical). We can't know this information since it is server-side only.

Well, not with that attitude! Perhaps by analyzing a large (i mean large) set of items we can approximate the probabilities? might not be worth the time or effort, but it's a thought

You are absolutely right. What I meant is that you can't know "for sure", but you can certainly create an approximate model with a large enough sample size.

Of course, you need to keep in mind that your samples will be largely skewed towards DPS (for weapons) since people won't post crap DPS weapons on the AH, so you cannot calculate the probabilities of damage-related affixes in your overall model. But if you are only looking at non-DPS affixes, then you should be able to tell if some show up more than others.