In Mandarin Chinese, zenme-interrogatives serve a dual purpose. First, they can be employed to inquire about the precise cause or reason for an occurrence, as in the case of causal zenme (“how come”). Second, they can be applied to seek information about the specific method for performing a task, as in the case of manner zenme (“how do you”). Drawing on data from Taobao (a prominent Chinese online shopping platform), this study investigates the application of causal zenme in the formulation and negotiation of customer complaints during Chinese e-commerce service encounters. It also conducts a nuanced analysis of the normative practices that underlie customer complaints. The analysis reveals that zenme-interrogatives are frequently employed to highlight deviations from normative business practices at the epistemic and deontic levels. At the epistemic level, these interrogatives are used to challenge agents’ asserted epistemic authority, or they point to agents’ disregard for customers’ entitlement to necessary information. At the deontic level, they are used to infringe upon customers’ deontic authority in receiving compensation or benefiting from other remedial actions. Overall, the findings contribute to our comprehension of how zenme-interrogatives are utilized to convey negative evaluations and emotions rather than simply solicit information or explanations.