It's remarkably difficult to find sources seeing as searching for 'Koyaanisqatsi' and 'Philip Glass' brings up a ton of renditions!

But the story goes something like this: He turned down the request to score the film, because he didn't consider himself a composer of film scores. So, the director went and got a load of Philip Glass music and overlaid it on the film himself and sent it to Glass. Once he saw it, he was in.

Nothing particularly mind-blowing in terms of a tale of how the film got made, but it could have easily gone in a completely different direction and not be the film we know.