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The latest El Niño event in 2009–2010, which is classified as warm pool El Niño, holds a unique ground in that it marks the strongest warming signal in the central Pacific but rapidly decays to strong La Niña. The strong eastward‐propagating cold anomaly at the subsurface level is found to be a key factor, and two possible mechanisms are suggested that resulted in the fast phase transition of the 2009–2010 event: 1) The anomalously warm Indian Ocean induces a surface easterly over the western edge of the Pacific, which generates the forced Kelvin waves. 2) The record‐breaking high sea surface temperature in the central Pacific excites a strong Rossby response that is to be reflected as an upwelling Kelvin wave at the western boundary. The strong subsurface anomaly then propagates eastward and results in an unusually fast phase transition of the 2009–2010 warm pool El Niño event.