In simple terms, an acquisition usually happens when a successful (read: low-risk) company purchases an unsuccessful (read: high-risk) company. It can be viewed as a transference of risk from the acquiree to the acquirer, and all things being equal, the market demands higher returns from riskier assets. In the short-term this is effected through the price, and one channel for this is that risk-averse holders of the acquiring company will move to safer harbors. The acquirer's stock thus falls, implying a higher return in line with the new level of risk. The market also demands lower returns from lower-risk assets, and so more investment will flow into the acquiree, driving its price up.
Obviously, there's more going on and this is not investment advice. It's just a rule of thumb.