Metathesis is the name of a process in language whereby the position of (typically) two sounds is inverted so that the historically preceding sound in a sequence exchanges places with its succeeding companion. In the history of English, this process is largely confined to sequences involving a liquid (l, r), as in third compared to three, wherein the vowel and the liquid have exchanged places in the ordinal numeral.
An example of metathesis that can be heard in Southern American English (but not only; cf. especially the speech of African-Americans, regardless of region) is [æks] instead of standard [æsk] for ask. (Etymologically, this verb goes back to Middle English asken, from Old English ācsian, āscian.) Judging by examples of metathesis in this word attested from Old English (e.g., in Chaucer), this is a very old change in the language, and reflexes of both forms can be found dialectally in both England and America.
How to explain the persistence of the non-standard form in American English? As in all cases of change, one must look for a rationalized explication of variety, not just for an ad hoc phonetic epiphenomenon or vagary of pronunciation. In semiotic terms, this instance can be explained as an implementation of a general principle governing the order of elements in grammar, whereby (ceteris paribus) the unmarked term precedes the marked term. Thus the etymologically correct order of the two consonants in the cluster in question, s and k, happens to contravene the normal cooccurrence of unmarked element before marked, /s/ being marked for the feature of stridency and /k/ being unmarked. The reversal of their order by metathesis in [æks] can thus be explained as a reversion to norm as applied to the sequence of markedness values in the consonant cluster.