Order by values with most incidence
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 ; 


The second part of the order by needs to be customerID just in case of ties. I assume  that if you have two or more customers with the same number of events, you don't want their events mixed up.