This is normal activity and some of the proceeds from these sales are usually used to pay taxes. According to The Farlex Financial Dictionary, SEC Rule 10b5-1 allows an employee of a publicly-traded company to create a plan of when and how to sell shares in that company. The employee writes and files this plan while he/she has no nonpublic knowledge about the company. Following this plan and selling shares according to it allows the employee to sell shares without fear of being accused of insider trading, regardless of what nonpublic knowledge he/she may later come to possess.
Apple COO Tim Cook, was recently awarded $17 million in stock options. He sold 300,000 shares, each worth $72.01 when granted, for $230 per share, generating $68.8 million, $32 million of which will be paid in taxes.
Chief Financial Officer Peter Oppenheimer sold 200,000 shares for $45.98 million, retail senior VP Ron Johnson sold 200,000 for $45.95 million, and marketing senior VP Phil Schiller sold 150,000 shares for $34.47 million.
Senior VP’s Bertrand Serlet, Robert Mansfield, and Scott Forstall also exercised their options.
The SEC Form 4 documents hve been posted online by Apple here.