Something to take note of is that Dec '15 opex closed well below max pain which might've been caused by delta hedging (and SPY trading ex-div). It's possible that further delta hedging the coming weeks puts selling pressure on the markets.
Option Expiration cycles http://www.investopedia.com/articles/optioninvestor/03/090303.asp
Max pain / strike peg http://www.investopedia.com/terms/m/maxpain.asp
Strike peg tool http://www.optionistics.com/quotes/strike-pegger