“Why are millions of dollars worth of orders being left unpaid?”. With tweets like this questioning brands’ policies, activists advocating for sustainable fashion re-discuss material starting points that are assumed by fashion brands, who argue that they are sustainable because they care about their workers’ conditions. This paper argues that activists use tweets to open _subdiscussions on material starting points_ to engage citizens and consumers, re-discussing factual _data_ that brands take for granted, such as the fact that they provide fair (...) conditions for their garment workers. Activists justify their opening of subdiscussions, often through an argumentative pattern that includes an argument based on the _locus from effects to cause_. They argue that if there are negative effects, the brand cannot claim to care about the conditions of its workers. In discussing how subdiscussions are used by fashion activists, this paper also introduces a conceptualization of Twitter argumentation as a discussion that is not isolated, but is part of a polylogical argumentation that takes place in different venues. For this reason, the argumentation used in tweets is reconstructed as a response to a fashion brand’s communication campaigns around sustainability, which extend beyond the confines of Twitter. As an empirical illustration, this paper is based on the campaign targeting fashion retailer Primark; the dataset includes the brand’s website as well as activists’ tweets. (shrink)