this is going to sound racist as fuck, but why do some black people (i only say black people because i've never seen/heard a non-black person say it) say "aks" instead of "ask?"
It's a common 'error' that can (and has in other cases) become common usage*. Funny thing is, "aks" or "ax" is closer to the original pronunciation before the 'wrong' and now 'right' variant "ask" became dominant. Which is why 'wrong' pronunciation and language use by extension is a gigantic crock of shit. Today's 'right' was yesterday's 'wrong' and vice versa. Which doesn't keep me from hypocritically being a pedant about certain things, even though I know that it's all normal. Just keep in mind that they don't do it because they're idiots.
So why does it happen? Well, it's not necessarily the same reason, but it's easier to say certain words a certain way because your speech apparatus is configured for your accent's patterns through years of use. Your tongue anticipates where it needs to be for the next sound and moves into position before you're even uttering it. Some British dialects' speakers for instance will render "this year" as "thi
sh year" because the way they pronounce "year" demands their tongue to shift into a position that clashes with "s" sounds as it does so. If I can find a clip of Jason Plato (a British race car driver) I'll put it here, because he does it very reliably.
Tongue twisters rely on this to work. Same issues crop up when pronouncing foreign words, or a different dialect of English. You will screw it up until you're used to the different patterns, because your speech apparatus moves subconsciously. If you ever meet a German who doesn't sound like they have an accent, ask them to say "squirrel" or "with which we". They might fuck it up no matter how good their pronunciation is otherwise.
*For a bit of jargon and many more examples:
http://en.wikipedia.org/wiki/Metathesis_(linguistics)#English