During the last couple weeks of the calendar year, there are always a large number of cross-currents impacting stocks that have little to do with their fundamental prospects. There are tax planning, portfolio moves, and pension plan reallocations that will cause some random movement. Also, there is quarterly option expiration and rebalancing of major indices.
Tomorrow is Quadruple Witching when equity and index, futures and options expire. In addition, there is preliminary additions and deletions to the Russell Indexes as well as rebalancing of the Nasdaq 100 (QQQ) , Nasdaq Biotech Index and the S&P 500, 400, and 600.
When rebalancing takes place the stocks that now have bigger weights due to price appreciation will be reduced and those that are down the most will have weights increased. This means that stocks like Apple (AAPL) and Microsoft (MSFT) , which are up big since the last rebalancing, will likely see selling at the close. Also sectors like banks, biotechnology and semiconductors will see some pressure.
This rebalancing action is well-anticipated and is gamed in a number of ways by sophisticated computer programs so it is a simple matter to trade. Just be aware that there is likely to be some very unusual activity when quite a few large blocks will cross at the close.