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It seems you don't really need the "frequency" for the sorting, only the "maximum frequency" per customer. You can use the COUNT(*) OVER (PARTITION BY customerID):

SELECT
    customerID, eventID,
    ROW_NUMBER() OVER (PARTITION BY customerID ORDER BY eventID) AS frequency,  
FROM 
    customerEvent
ORDER BY 
    COUNT(*) OVER (PARTITION BY customerID) DESC,
    customerID,
    frequency DESC ;