Over the last decade, European Union (EU) trade agreement negotiations in the form of the Transatlantic Trade and Investment Partnership (TTIP) and the Comprehensive Economic and Trade Agreement (CETA) with Canada have been strongly contested. By contrast, many other EU trade negotiations have sailed on with far less politicization, or barely any at all. In this contribution, we assess a series of plausible explanation for these very varying degrees of politicization across EU trade agreement negotiations—conceived of as the combination of polarization of opinions, salience given to them in public debate, and the expansion of the number of societal actors involved therein. Through a review of existing explanations, we show how each of these explanations faces a set of challenges. In the third section, we argue it is useful to conceive of these existing explanations as structural background conditions enabling agency on the part of interest group and civil society organizations. We therefore close by sketching how literature on the relationship between interest group mobilization and public opinion could inform further comparative research on trade policy negotiations, and on politicization of EU policy making in general.