A few hypotheses:-
Some of them are phonetic approximations of words in other languages. e.g. Shool Aroon.
Some are part of the stylistic fashion of the day. e.g. "Boop-a-doop" in the twenties, adding an "O" to the end of words in folk, or "Yeah yeah" in the sixties.
Some are an attempt to convey an instrumental accompaniment. e.g. "Whop bop-a-loo-mop a lop bam boom" (Little Richard's indication of how the drum fill should go)
Some are vocables- sung syllables with no meaning. e.g. the "hey yah hey ya" used in a lot of native American chants, in Scat singing and Mouth Music.
It's down to individual cases after that.
Cheers
Dave