Apple's stock can't seem to catch a break lately.
Shares are off another 4 percent today, and Apple has lost a fifth of its market value since shares peaked in mid-September. It closed down 6.4 percent to $538.79, but is up slightly in after-hours trading.
Over the past few months, Apple has faced questions about whether it can maintain its current dominance in the mobile industry, with its iPhone and iPad both ceding market share to devices running on Google's Android operating system. At the same time, investors are fretting over the decline in profit margins at the company, which executives believe is a temporary hiccup spurred by the introduction of so many different products.
There are myriad reasons for the stock decline today. StreetInsider said that clearing firm COR Clearing raised its margin requirement on Apple, citing an already high concentration in the stock. That likely rippled out to other firms looking to reduce their heavy positions in the company.
Apple was also likely hurt by an IDC study issued today that has Apple ceding share in the tablet market -- even as the market itself continues to gain momentum. Much of the growth in the market is expected to stem from lower-cost and smaller tablets. Though Apple has a presence there with the iPad Mini, the forecast suggests its core iPad business could face pressure.
Piper Jaffray analyst Gene Munster, meanwhile, said some investors may be reading too much into a Digitimes article that mentioned a decline in demand from Apple on components, which could be seen as a leading indicator for a weaker calendar first quarter. Munster said the decline in demand is typical after the launch of a new product, and isn't an accurate way to judge how the March quarter will fare.
In addition, some investors may be concerned that China Mobile will be selling the Lumia 920T, announced today by Nokia and the Chinese carrier, rather than the iPhone. The iPhone 5 has shown up at China Unicom, but has yet to launch with China Mobile. Munster said he believes the phone will make its way to the world's largest carrier by customer base by the end of the year.
Fortune believes that some investors, particularly large funds, pulled out of Apple's stock after the company indicated it would not be paying out a special dividend, which other companies such as Wal-Mart has issued over the last few weeks.
Apple's shares did pop briefly in late November as the company improved its supply of iPhones and entered the crucial Black Friday-Cyber Monday shopping frenzy. Several analysts noted that iPhone production yields have improved, which could lead to better sales. But the stock's return to decline this month suggests there remains some skepticism and concern over Apple's next big growth driver.
Updated at 11:02 a.m. and at 1:24 p.m. PT: to include additional background, analyst comments, and the closing price.