This seems to be a common (though not universal) feature among verbal autistics (and perhaps for non-verbal autistics, too, in writing); we just have so much going on in our heads that it is difficult to condense it all, so we share it as it is .
Are you familiar with the proposed four "types" of autism (aloof, passive, active-but-odd, and stilted)? Rambling would be more common in the active-but-odd group, a supposed group that wants to communicate and interact with others but does so in an unusual manner, whereas those in the aloof group supposedly pay little mind to the outside world and thus does not interact much with others, those in the passive group are supposed more reversed and, well, passive, and those in the stilted group are supposedly over-formal and follow what social rules they understand rigidly (although I suppose that one can ramble in an over-formal manner). It's a controversial and perhaps inaccurate theory, as many people seem to fall in multiple sub-types, but it's interesting, nevertheless.
Though I do ramble on occasion, I seem to be passive by nature and stilted by molding and, thus, am more "scripted" and reversed with my patterns of communication, which often does not translate into rambling (though my posts on WP many seem contradictory to this ).
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I am not a textbook case of any particular disorder; I am an abstract, poetic portrayal of neurovariance with which much artistic license was taken.