Because useless anal-cysts said they'd sell 55 million iPhones, not 51m. They also claim they're not branching out enough, so that's another reason.
I have to say, whether you're talking Apple, Google, MS, or whoever, "analysts" are friggin' useless, IMO. They truly seem to pull numbers out of their asses, and when the company in question doesn't meet their expectations, the gossiping school girls known as the stock market punishes the company. Meanwhile, a company like Amazon, who continues bleeding money, has their stock go up, simply due to speculation about the future, and expansion.
But WTF do I know, I'm just a dumb IT guy.
EDIT
I also hate shithead rich douchebags like Carl Icahn, who don't give two fucks about the actual products/services of the companies he invests in (Apple, Dell, Netflix, etc), and just does it to strong-arm companies into doing what he wants to make a quick buck.