The phrase "turn down" is the opposite of "turn up". To "turn down" would be to decrease the intensity of the party. And "turn down for what" means something like "don't stop the party for any reason".
But in my experience, party intensity and music volume are generally correlated, so you would probably turn down the former by turning down the latter.
As described at Wiktionary [0] - it's an idiomatic way of saying that you're going to lower the volume through use of a control to do that. The context that was used has nothing to do with party.
[0] https://en.wiktionary.org/wiki/turn_down
EDIT: My bad, thought it was in response to...
> I guess it will mostly reflect the musical taste of assholes who turn their music up loud. Hmm, but maybe all culture works like that.
> At its core, turn down for what is a phrase used to promote having a good time. The phrase itself implies that there is no reason to turn down and stop partying.