This is actually a matter of general reference that should be easily explained by consulting any dictionary worthy of being called such — and certainly can be found in The Dictionary, if one would but look.
But I will explain it anyway. :)
English has at least 6 different prefixes, a-. This, however, is not one of those six. Rather, the one you see in knees all a-tremble is actually an old preposition that we no longer use, just as it is in here we come a-wassailing, in nine lords a leaping, and in twice a day.
It was a variant of the preposition on that lost its -n. The OED explains in a prep. (which is just the 8th entry in the Dictionary):
Variant of on prep. with loss of the final consonant -n, reflecting an unstressed pronunciation of the word in proclitic use; compare an, variant of on prep., and also o, variant of on prep. Compare a- prefix3, away adv., aright adv.
The loss of final -n in this word occurs early in terms of the developments described at N n. and perhaps began in fixed idioms where the word was felt to be almost a prefix; compare the parallel development represented by a- prefix3. A following consonant favoured the loss of final -n (compare discussion at N n.) and so until the 17th cent. the word was often found in complementary distribution with an, variant of on prep., which was common before vowels (with variation between the two before following h-).
The separate preposition a ceased to be used in standard English after about 1700, being replaced by the full on, in, or the various prepositions which represent them in modern idiom, surviving only in a few set uses from branch II., such as to go a begging, to set a going (occasionally, before a vowel or h-, in form an, after an, prevocalic variant of a adj.; compare quots. 1759, 1780 at sense 11b), and in temporal distributive phrases, as twice a day, once a year, where it had been early identified with the indefinite article (see a adj.4). It also survived in a large number of combinations, where it was treated as a prefix to the governed word, and the whole as a compound adverb.
As I said, it’s in the Dictionary.
Were you to look there, you would also see that although there is some connection between askew and askance, and several other, less common words besides, the actual origin is obscure. The Dictionary says of askance adv.2:
There is a whole group of words of more or less obscure origin in ask-, containing askance, askant, askew, askie, askile, askoye, askoyne, (with which compare asklent adv., aslant adv., asquint adv.,) which are more or less closely connected in sense, and seem to have influenced one another in form. They appear mostly in the 16th or end of the 15th cent., and none of them can be certainly traced up to Old English; though they can nearly all be paralleled by words in various languages, evidence is wanting as to their actual origin and their relations to one another.