Interesting! I never knew bolero was a whole genre, I only knew it as a particular piece by Ravel.
A Google search for bolero has an entry:
What is bolero famous for?
Boléro, one-movement orchestral work composed by Maurice Ravel and known for beginning softly and ending, according to the composer's instructions, as loudly as possible.
I once lived just far enough from my workplace that the drive time was the same as the Ravel piece. The way it builds over the 15 minutes coincided nicely, and effectively, with waking up by the time I got to the office!