The rise of online social media has fostered increasing instances of deviant online behaviour. One of the most lethal is collective bullying i.e., trolling, which has severe impacts including suicides of victims. Yet, it remains a mystery what kind of factors lead social media users to engage in trolling. To explain social media trolling, we contextualized concepts from deindividuation theory. Using fuzzy-set qualitative comparative analysis technique to analyse survey data from 337 Facebook users, three configurations explaining social media trolling have been developed. The results suggest that social media affordances and deindividuation states together give rise to trolling. Our results offer theoretical and practical implications.