In stock trading, we say that there are a million reasons to sell and only one to buy. Sales aren't a particularly good signal, since they can be very specific to ones' personal situation.
Well, you think you can't get a better return on your investment elsewhere.
Just going up isn't enough if something less risky goes up faster.
Going down a little can even be a relatively good bet if everything else is absolutely tanking. Losing 1% of your principle is a pretty good deal if the stock market (and the dollar) go down a lot.