From Ballentine's Law Dictionary, 2nd edition (1948):
bozo. The word does not appear to have acquired a settled meaning recognized by lexicographers, but it is safe to assume that it has a meaning similar to "rat" and "scab" as all three words are used in connection with labor troubles.
Maybe Wizzo the Wizard was employing black magic to keep the pesky International Brotherhood of Clowns Local No. 178 away from the set.