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. If you have two or more customers with the same number of events, then I assume you don't want their events mixed up.
