Why aflame is not at all to flame as asexual is to sexual:

Prefixing a- to verb forms ending in -ing, as in a-hunting and a-fishing, was once fairly common in vernacular U.S. speech, particularly in the highland areas of the South and in the Southwest. Such verb forms derive from an Old English construction in which a preposition, usually on, was placed in front of a verbal nouna verb to which -ing had been added to indicate that the action was extended or ongoing. Gradually such prepositions were shortened to a- by the common linguistic process that shortens or drops unaccented syllables. The -ing forms came to be regarded as present participles rather than verbal nouns, and the use of a- was extended to genuine present participles as well as to verbal nouns. Eventually a- disappeared from many dialects, including Standard English in the United States and Great Britain, although it is still retained today in some isolated dialect areas, particularly among older speakers. Today, speakers who use the a- prefix do not use it with all -ing words, nor do they use it randomly. Rather, a- is only used with -ing words that function as part of a verb phrase, as in She was a-running.

PS: An acre is 160 square rods. 'Square rod'. Now giggle. I command that it be so.