Wasn't that due to the Chinese New Year last year?

You shouldn't compare Q3 2015 to Q2 2015, but to Q3 2014. Otherwise seasonal effects and things like product launches play too big a role. The iPhone 6 has been out for some time, for instance. Sales of that taper off a bit after release.

"Greater China sales fell by 21% from $16.8 billion to $13.2 billion"

That's reading sequential quarters, which pretty much nobody in finance (who wants to be taken seriously) does, because it overlooks seasonal factors. The year-on-year figure (which serious financial people do use) is up over 100% from $5.9bn. The stock market maybe is concerned that China is too important and that if its stock market is plunging, so will sales of Apple stuff.