Because this was supposed to be their holy grail quarter, you know the one where they had just released a new phone (instead of the 4s "refresh") with LTE and a big screen and it was going to show that Apple was going to gain against android, that it could compete, at least in this quarter, substantially. Over the course of the quarter it became apparent that they were selling a lot of iPhones, just not selling a lot to new users. Even ATT had around 80-85% of iphones sold were to existing iPhone customers. This is essentially a loss for ATT because Apple subsidies are staggeringly high, almost double in many cases. It's a good deal for Apple that many of their users are loyal, but without significant marketshare growth there is concern.
So Apple managed to move 47 ish million iphones, meanwhile Google activated around 150 million android devices in the same quarter. This was Apples quarter, their new, large screen phone, LTE, not a refresh, and the wind fell out of the sails.
At this point the analysts start to wonder if the shiny paint that most tech sites have been talking about disappearing for a year or more is starting to fall off of the iPhone and it's long term growth prospects is probably not nearly as good as some had previously believed.
Couple that with the absolutely dismal sale of Mac computers, and the eroding tablet marketshare to android and that why investors are taking a step back.