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It's unclear exactly why the stock is crashing, but it's part of a trend that we've seen since the end of July after Apple reported earnings. While there's no clear reason for the drop, it comes as investors are expressing concern about the iPhone and Apple Watch.
KGI Securities recently issued a note saying that iPhone growth is expected to remain flat or go negative in the fourth quarter of 2015. This was partially attributed to the trouble with China's economy and the idea that Force Touch may not be a big enough hit with consumers to drive iPhone growth.
At the same time, UBS also sent out a note indicating that there's "lackluster interest" in the Apple Watch compared to previous Apple product launches based on search interest. (That being said, UBS also found that Apple Watch interest has been strong enough to impact traditional watch sales).
Apple shares have been falling since the company reported earnings on July 21. Stock was down 4% one week ago.
Despite Apple shares being down, there's reasons for investors to be optimistic. As the company reiterated in its earnings report, Apple is a strong business that's pumping out billions in cash. Although iPhone unit sales fell below expectations, the iPhone business overall is up by 35% year-over-year.
Here's a chart from Yahoo Finance: