My guess is that the knobs are pull for on and push for off is that Organs with stops work the same way. My mother-in-law has one of those antique reed organs that you pump with your feet, kinda like the way the old sewing machines worked. It has reeds and stops and is really kinda like a great big accordion. And, if you wanted certain reeds turned on, you pull out the stop.

This motif even shows up as the standard metaphor: "Pull out all the stops". It is not "Push in all the stops".

Which isn't to say that it can't be the other way, and there are some accordions that are built the other way. So the choice seems to be arbitrary. I'm just saying that is maybe where the convention may have come from.

-David