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Abstract The episodic stranding of millions of pelagic red crabs (Pleuroncodes planipes) along California beaches is a striking and puzzling phenomenon.Pleuroncodes planipesare usually abundant off Baja California, Mexico, and their appearance in central California is thought to coincide with anomalously warm waters and northward advection related to El Niño. This anecdotal association has stimulated many hypotheses, but no hypothesis has gained clear and convincing support. Motivated by an unprecedented number ofP. planipesstrandings and at‐sea observations in 2015–2019, we compiled 10P. planipesdatasets spanning 1950–2019, tested the anomalous advection hypothesis using a transport analysis from 1981 to 2010, and explored other compelling hypotheses. We foundP. planipespresence off central California was related to anomalous advection of waters from Mexico, sometimes but not always associated with El Niño events, withP. planipeslikely remaining residents of northern waters for several years without additional anomalous transport but potentially facilitated by warm waters. By identifying a mechanism behind episodicP. planipesrange expansions, we show that a source water index could provide an early indicator for anomalous events in the future.