There have been lots of research papers and books done on the subject of seasonality. Michael O'Higgins even mentions it in Beating the Dow . The most frequently cited reason is tax-loss selling. As high-income people come to the end of the year, they start thinking about their tax liability.
Many mutual funds report the results and details of their portfolios on an annual basis.
There is also a tendency for fund managers to dress up their holdings by buying winners. When that year-end report comes out, it looks good to have last year's big gainers on the list of holdings. This might also contribute to a slight rise in price for some of the stocks you are about to sell. Many traders go on vacation around this time. Most sell all their positions before leaving on vacation.