Many traders and large hedge funds trade strictly in merger arbitrage. These traders look at deals that have already been announced, and examine the deal terms and the levels the stocks are trading. If they think the chances of the merger going through are great, but the stock being acquired is trading below where it “should,” they see opportunity and will often buy shares (or call options or take some other bullish position). In the case of Pfizer-Allergan, there was plenty of “opportunity.”