I am guessing some big boy decided to unload today.
The sizable decline started at 1:15pm. The stock goes from zero volatility for the day, to a sharp decline. Looks like someone wants to minimize a position or get out before earnings.
Today is option expiration, maybe that may have something to do with it too. Maybe an effort to wipe out value of some of the call options.
I do not know the specific reason, but I do know a few things.
1. In a 3 month time frame, AAPL has been flat. (The stock is where it was when it reported last quarter. So the market is saying, AAPL generated ZERO value within the three month time frame it has gained market share from Android.)
2. Apple has generated value in the last three months. It has grown its computer business while the industry has shrunk and increased iOS market share against Android while maintaining high price points.
Despite the volatility within the stock, at 420, the market is ignoring the value generation. (As for future prospects, iPhone 4S just started selling in China, iPad 3, iPhone 5 LTE and Apple TV on deck for 2012.)