Via Matthew Ford, author of War in the Smartphone Age and other books, the Israelis had, in addition to having hacked one of the most popular apps in Iran, gotten intel that Khamenei and other high-ranking Iranian government officials would be meeting in a specific place at a specific time, so that's why the snap launch of a decapitation strike.
EDIT: According to an NYT piece, CIA's trying to claim credit for the intel.
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