Because the large amounts of open interest in the call and put options at a particular strike will cause market-makers who are usually short-vol to have to buy or sell stock to close out their positions.
Magic or manipulation, take your pick. Or maybe it the gravitational pull of those near term strike prices. No matter, options are a sucker bet.
See where the money is going and pick your side. Takes research plus i don't think anyone will give away their work for free.
1. There have been many, many studies done on this subject. The correct conclusion is that there is not a systematically exploitable bias to "pin". There are certain sectors of the market where you can trade it, but it tends to be in stocks that are on the illquid side ... so your poker skills had better be sharp. 2. Much more often than not, when a stock DOES pin, it's due to the fact that the market makers are LONG the strike. Their hedging of their position as expiry approaches makes them buyers of stock below the strike, and sellers above.