In California’s Franciscan mélange, we studied why some slopes move slowly as earthflows while others collapse suddenly in massive landslides. Comparing the Eel River canyon and the Big Sur coast, we found that the speed of erosion is key. In Big Sur, fast coastal erosion creates steeper slopes where slow slides can quickly turn into dangerous collapses. This happens because rapid erosion lowers the water table and changes slope stability. Our findings show that faster erosion can increase the risk of sudden, destructive landslides.