Mystery: Why is Apple having such a bad week?

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Why is Apple underperforming so much after it announced seemingly positive first weekend iPhone sales?

That is the big question asked by many investors with Apple shares down 4.5 percent week to date compared to a flat market.

"Sales for iPhone 6S and iPhone 6S Plus have been phenomenal, blowing past any previous first weekend sales results in Apple's history," said Tim Cook, the tech giant's CEO in the Monday morning press release.

The company reported it sold more than 13 million of the new iPhones during the first three days after launch compared to 10 million last year.

CNBC Pro asked several Apple experts and traders to find out why the company is selling off. Here is what they said…

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