The problem here is that the stock has been over inflated for a long time due to surpassing analyst estimates year after year due to iphone growth. Investors are realizing the real threat of android that is stealing iphone market share..
Dont forget apple has always been a niche player. Thats how they are able to sell devices with large margins. Previously they were the leader in mobile devices but this has changed. They will never attain the previous level of market share. Investors are not that confident in apple's ability to continue major innovation without steve jobs.
Therefore that means lesser earnings (relative) in the future, which means a correction in their stock price.
The stock has been artificially inflated for too long and this was bound to happen some time.