Today's reason seems to be tepid Chinese pre-orders for the new iPhones, plus news that subsidies for the iPhone in China will shrink.
From the minute Apple announced the price of the iPhone 5C - $99 with a two-year contract - the stock has dropped 10%.
The stock didn't immediately drop on the price announcement. The stock started really fading after Apple's iPhone event ended.
Investors didn't like the iPhone 5C price, which is $549 off contract, which means Apple didn't do anything different with iPhone pricing this year. They also worried that Apple didn't announce a deal with China Mobile.
The bottom line seems to be that investors are worried about the growth of Apple's iPhone. No one thinks it's going away, but if the growth stagnates, then it's a problem.
Yahoo Finance screenshot