Despite substantial investments in business process management (BPM), every organization experiences deviant processes, i.e., processes that show different behavior than intended. Thus, process deviance is an essential topic of BPM research and practice. Today, research on process deviance is mainly driven from a computer science perspective. IT-based methods and tools (e.g., deviance mining and prediction or compliance checking) detect process deviance by comparing log data from past process instances with normative process models or execution traces of currently running instances. However, requiring process models and event logs as input, existing approaches are expensive and limited to processes executed in automated workflow environments. Further, they can only detect process deviance, not explain why it occurs. Thus, knowledge about reasons for process deviance is immature. What is missing is a systematic exploration of reasons for process deviance. Against this backdrop, we compiled and structured reasons for process deviance based on a rating-type Delphi study with more than 30 experts from industry and academia. Thereby, we chose a process manager’s perspective as analytical lens, as process managers are familiar with and responsible for business processes end-to-end. We also analyzed the reasons’ importance for causing deviance in routine and nonroutine processes, two process types that capture the nature of processes in terms of variation and variety. Our results contribute to the descriptive knowledge on process deviance and serve as foundation for prescriptive research.