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.