It comes down to this: most shares of Apple are being held by large institutional investors, and they have guidelines on how much of their portfolio can be consisted of a SINGLE stock. Before the drop, Apple has grown almost 70% this year, which has made it a huge portion of a lot of investment institutes' portfolio. Selling some of the shares for a profit-taking and re-balancing of position is what happened next. That's exactly what happened back in May when the stock dropped close to 20% as well.
Now you may ask why it has dropped close to 30% this time? Simply because the market in itself was not doing well during this past month either. After the usual 20% correction, the market was hit with Obama's reelection, the fiscal cliff uncertainty, uncertainty on next year's capital gain tax increase, plus the usual end-of-year-profit-taking. So the correction went quite a bit further than those investment institutions have expected. That is why pretty much all analysts on Wall Streets right now are EXTREMELY bullish on Apple, since those fund managers will inevitably use this opportunity to restock (pun intended) their portfolios with cheap AAPL shares in the near future.
And don't listen to anyone who tell you Apple is "over-priced", right now it has a lower P/E ratio than Microsoft, so unless you really believe it has lower growth potential than Microsoft in the near future, the stock is quite undervalued. The stock has outgrown MS, Google, etc by quite a bit in recent years, but Apple's bottom line have grown by even more.