Apple's valuation is far from reaching the weight limit on the NASDAQ yet, though, and the index gurus have repeatedly said it doesn't need any rebalancing just yet, so there might have been other factors in play.
One of these factors that might have spooked Apple's investors, with its shares the poster child of many ETFs, hedgies and pension funds, is the trademark saga with Proview in China. Authorities are banning iPad sales in a few cities, and Apple itself asked the local branch of Amazon to stop offering iPads from other retailers until its appeal verdict comes in.
China is a market of huge and growing importance for Apple, so any hiccups there are likely to spook investors a bit, despite the fact that Apple doesn't make nearly as much from the iPad compared to the iPhone golden goose. Add that to the fact that some people just wanted to take some profits after Apple's nice ride to $500 a share in the past weeks, with automated orders to sell a bit of shares when they hit that threshold kicking in, and the rare 2%+ slide in Apple's shares makes much more sense.