The poem's sound is like an early twentieth century version of freestyle rap. Ever heard a freestyle about apples before? Us neither. That's why Frost is an American original. "After Apple-Picking" has an improvisational feel, and Frost doesn't worry about maintaining a regular rhythm or rhyme scheme. He knows that the audience isn't going to quibble with the lack of a symmetrical form if the total package sounds great.
The rhyme scheme is the best example of how Frost is motivated by sound and not by form. He's like a rapper or a slam poet in the way that he sticks with a rhyme until he runs out of good uses for it. In lines 14-16, he rhymes "well," "fell," and "tell" within a short space. Or he'll wait several lines before rhyming with an earlier word. Several of the rhymes are spaced four lines apart, like "earth" and "worth" (lines 33 and 36). And if he sees the chance to use a rhyme right away, he'll just cut a line short, as with lines 31 and 32, where "fall" is followed immediately by "For all." In short, this poem would still sound great if spoken as a freestyle rap: give it a try!