Taking tax losses. Rebalance your portfolio. 去糟粕，存精华。
The reason you might want to do so early, though, is to get a jump on everyone else doing the same thing.
Another advantage is that if you want to take losses but still own the stock in the future, you have to wait 30 days after selling to buy it back. If you sell now, you can buy back in November or December, just as latecomers are selling, hopefully pushing the price down to let you get in more cheaply.
