I'm thinking that it was in 2010 when Cook was granted 75,000 shares. I don't know if it was restricted stock, and if so, when it vests. But as part of his 2011 pay package, he was granted 1 million shares, of which, half vests in 2016 and the balance in 2021. But even with this sale of 20,000 shares, he still has almost 14,000 shares left.
So he's either already sold a big chunk of the 75K that he was granted in 2010, and now he's down to about 14k, or it hasn't vested yet and he decided to sell a chunk of what he had on the books now (for whatever reason), knowing that he's going to be one of the top individual holders of AAPL soon enough anyway.
As for reasons, it could be anything: taxes, wants to (slightly) diversify his portfolio, wants to buy a nice Gulfstream, going to hire Kim Kardashian to dress up in a gorilla suit for his next birthday party... could be anything.
But unlike some of the foolish, uninformed comments by some of the people on the linked site, there's nothing to be concerned about.
If two people always agree, then one of them is redundant.
If two people always agree, then one of them is redundant.