I think the sell off is based on iPhone sales figures. I read an article that I think is scaring investors, and it was the ending of subsidies from wireless companies.
Apple so far has been able to get away with higher subsidies for it's iPhone than any other phone out there because they've shown that iPhone is in-demand. As soon as there are any indications that people are moving away from iPhone and are looking into other options, the ability to strong arm a wireless carrier into higher subsidies, or bigger bulk purchases of iPhones for a cheaper price, dwindles.
Verizon Wireless just started carrying iPhones not too long ago, and they've usually enjoyed profits on selling their phones, but with the iPhone they've seen that profit go down significantly. Google is rumored to be releasing their first Motorola phone with the same concept it has with the Nexus phones, and that is bringing the off-contract price of the phone down. If Google were to release a phone for Verizon at $350 retail price, that is $300 LESS than a new generation iPhone at retail price. Those numbers would entice any carrier. Google is rumored to be bringing this device to all carriers too at that low price.
So to sum it up, if Apple can't leverage it's domination on carrier subsidy with strong demand for iPhones, then carriers will do away with subsidies, and will most likely either 1) Stop selling the iPhone, and let Apple sell it themselves or 2) The bulk order of iPhones will be at a much lesser price than Apple is currently demanding
Margins will go down, and Margins is basically what has kept Apple insanely profitable.