Investors are not always rational in the way they set expectations.
These irrationalities may lead to expectations being set too low for
some stocks at some times and too high for other stocks at other times.
Thus, the next piece of information is more likely to contain good
news for the first stock and bad news for the second. Price changes themselves may provide information to markets. Thus, the fact that a stock has gone up strongly the last week may be viewed as good news by investors, making it more likely that the price will go up today then down.