The 2018 summit and flank eruption of Kīlauea Volcano was one of the largest volcanic events in Hawai'i in 200 years. Data suggest that a backup in the magma plumbing system at the long-lived Pu'u 'Ō'ō eruption site caused widespread pressurization in the volcano, driving magma into the lower flank. The eruption evolved, and its impact expanded, as a sequence of cascading events, allowing relatively minor changes at Pu'u 'Ō'ō to cause major destruction and historic changes across the volcano. Eruption forecasting is inherently challenging in cascading scenarios where magmatic systems may prime gradually and trigger on small events.