Purpose -This study shows how customers engage in negatively-valenced influencing behavior (NVIB) and what triggers customers to use different forms of NVIB in an online context. Design/methodology/approach -A qualitative study is conducted using an unobtrusive netnography. Data collected comprises 954 negatively valenced online reviews posted on TripAdvisor to hotels, restaurants, and 'things to do' in twelve different destinations worldwide. Findings -Drawing on recent literature relating to customer engagement behavior (CEB), this paper identifies and conceptualizes the relationship between five cognitive (service failure, overpricing, deception) and emotional (disappointment and insecurity) triggers of six forms of direct (dissuading, warning and endorsing competitors) and indirect (discrediting, expressing regret and deriding) NVIB. Research limitations -The unobtrusive netnography has inherent limitations that lend itself to inductive rich insights rather than generalization. The study only focuses on NVIB within a specific online context namely TripAdvisor. Practical implications -This paper provides managers with knowledge of the specific triggers of NVIB. Additionally, the paper conceptualizes the various forms of NVIB, how customers use them and what triggers them to use each form. Moreover, the paper offers relevant data-inferred recommendations to service managers on how to manage each form. Originality/value -This research is the first to identify forms and triggers of NVIB, classify direct and indirect forms, and conceptualize relationships between forms and triggers.