Prepositions acquired a somewhat greater importance than they had had in Old English. Even in Old English times, on was sometimes reduced in compound words like abūtan (now about), a variant of on būtan ‘on the outside of.’
The reduced form appears in Early Modern English aboard, afield, abed, and asleep, and with verbal nouns in -ing (a-hunting, a-bleeding, a-praying). The a of “twice a day” and other such expressions has the same origin.