As a dad who has seen all but the worst of their works, and by that I mean Cars 2 and that it is their worst is just based on assumption, I can tell you by watching countless hours of special features that the best of the Pixar films are the ones they make for themselves.

It's perfectly possible that someone who is a lead animator, story artist, even the director themselves is just a big Mastodon fan. And they want nothing more than to find a way to bring their own love of music into this project. It may have nothing to do with Disney/shareholders/pandering at all.