There's also the Santa Clause Effect. Every year, investors take profits to afford to pay their taxes, pay for the holidays, etc. During that time, savvy investors take advantage of the lower prices to pick up stocks at bargain prices.
It's also end of the year/quarter, when many contracts/short sells are due.
During the holidays, many investors are not taking time to manage their holdings, so they divest the more volatile ones.
Oct-Dec is historically an unpredictable market period. People are looking for reasons to sell ahead of Christmas.