OSX updates are driven by iOS updates, which are the main revenue generators. Apple must release iOS every year to follow phone releases; OSX has to follow suit in order to support iOS "integration" features (Handoff, Photos etc etc).

Because release cycles are so short, test cycles are shorter as well; and of course new hardware is tested first (and foremost), so updates to older hardware will see more bugs, and updates are always buggier anyway (because it's harder for developers to predict the state of your system pre-migration).

I certainly see the yearly release cycle for OSX as being a big factor for the perceived fall in quality. In addition, there is probably a glut of "peak Air", people who switched to Mac when the Air was unrivalled and have not bought anything since. Their hardware is less and less tested with each update, so they're feeling the pinch. Apple don't care, because they want new money from them.

Are they really losing a significant percentage of their customer base due to unreliable software though? I doubt it. I expect that most people who use OS X and care about bugs probably have a negative perception of Windows (whether that's still warranted or not). So that leaves Linux. And as good as desktop Linux has become, it's still not the OS for people who don't want to spend time dealing with strange computer issues. So what's left to switch to?