I was reading something about songwriting forms, and thinking again about the rise and fall of the A-A-B-A form (A section, repeat of A section, B section in a contrasting key, and a closing version of the A section). In pop songwriting, this form appeared suddenly and disappeared just as suddenly.
Before the '20s, AABA was seldom used; popular songs were mostly either verse/chorus or A-B-A-C (or the A-B-A-C-A form heard in songs like "Alexander's Ragtime Band" and "Make Believe"). By the end of the '20s, AABA was the default form for popular songwriting, though ABAC was still used sometimes. "Ol' Man River," "Makin' Whoopee" and "The Best Things In Life Are Free" are examples of late '20s AABA standards written by composers who weren't using that form just a few years earlier. I've never been quite sure why AABA took over popular music so suddenly.
And then AABA more or less vanished from mainstream pop music by the early '70s, replaced by verse/chorus and other, earlier forms. I haven't seen a full explanation of the downfall of AABA; I used to think it was because of rock n' roll, but in fact the form was quite common in rock music for quite a while. But people who had used the AABA format a lot, like the ex-Beatles, sort of gave up on it. (There are a lot of Beatles songs that we don't even think of as being AABA that do, in fact, use that form -- like "Here Comes the Sun": the "A" section is the "Little Darlin'" section, and the "B" section is "Sun, Sun, here it comes".)
I'd be interested to see some popular-song expert -- which I am not, especially on more recent pop music -- explain the peaks and valleys of AABA as a songwriting form. One explanation that comes to mind, without knowing much about whether this is true, is that AABA was the perfect songwriting form for the single record release. With the rise of electric recording in the '20s (which allowed for better sound reproduction and made more people start to buy recordings, rather than sheet music, of popular songs), songs needed to be shaped so that they would work well on three-minute records. The AABA form has the perfect shape for a short record: the singer sings the song, the orchestra plays the first 16 bars, and the singer ends with a repeat of the last half of the song. It's a very satisfying format that provided the basis for the hit single for many, many years. With the decline of the single and the rise in the importance of albums -- where songs didn't need to be any particular length or shape -- the point of AABA was less clear and singer/songwriters reverted back to earlier forms.