Apple (AAPL) shares tumbled Wednesday as investors fretted about possible profit margin declines, market share erosion in smartphones and tablets, and the absence of the next big thing.

Wall Street also could be selling based on a technical analysis of Apple's stock, analysts say.

Apple shares fell 6.4% to 538.79, its biggest one-day percentage decline in nearly four years. Apple has been a volatile stock lately. Shares tumbled 28% from Sept. 21 to Nov. 16, after hitting an all-time high of 705.07. Apple then climbed 18% as of Monday, only to fall hard for two straight days.

The Nasdaq fell 0.8% on Apple's big loss, while the other major stock indexes rose modestly.

News reports related to Apple's iPhone and iPad businesses may have spooked investors.

An article Wednesday in Taiwan's DigiTimes suggested a 20% quarter-to-quarter decline in Apple's demand for iPhone components because of a coming "slow season" in early 2013.

But this is to be expected coming off the iPhone 5's launch quarter and may not indicate how unit sales will trend in the March quarter, Piper Jaffray analyst Gene Munster said in a research note.

China's Great Wall

Also, Nokia (NOK) said Wednesday that China Mobile (CHL) would launch its Lumia handset by the end of the year, sending the ailing handset giant's shares up 8% to their best level since April.

Apple has yet to get an iPhone distributed by China Mobile, the world's No. 1 wireless carrier with 703 million customers. Plus, China Mobile revealed Wednesday that it plans to develop company-branded handsets.

Some investors took those announcements to mean that China Mobile won't carry the iPhone, but Munster believes it will add Apple's smartphone in late 2013.

Meanwhile, market research firm IDC said Wednesday that Google (GOOG) Android-based tablets would rise to 42.7% of the global market this year from 39.8% last year. Apple's share is seen dipping to 53.8% from 56.3% last year. IDC said the market is shifting to smaller, cheaper tablets.

That could mean increased competition and cheaper tablets that pinch Apple's fat profit margins.

"Investors are sensitive to any data that might be negative toward margins," said Avondale Partners analyst John Bright.

Margin pressures also are looming over the iPhone business, Bright says. Some investors worry Apple might sell more lower-margin iPhone 4 and 4S models than the new iPhone 5.

Apple's renewed stock sell-off could be related to traders using technical analysis, Munster says.

"Apple's simple 50-day moving average is nearing its 200-day moving average, which is a negative technical sign," he said. "For this technical indication, most of the damage has been done to (Apple), but there could be a worst-case additional 10% move to the downside .

Apple's biggest issue could be coming out with its first new product category since the death of co-founder and CEO Steve Jobs, Bright says.

"Apple faces a problem in that it's a two-product company today," Bright said. "Innovation will be key to Apple's future growth." That could be the iTV.