Down is next because it's a list of songs with the first song at the top and the last at the bottom.
Frankly it's the orientation that makes the most sense when you consider it given most people will be listening from a streaming service, but back when CDs were a thing the songs weren't considered a list but tracks numbered from 1 to n. The up button incremented the track number and so it made sense for up to be next.
Going even further to tapes, fast forward and rewind literally moved the tape left to right/right to left, and so it made sense for them to be right and left respectively, however now it makes less sense other than being what older people are used to