Etymology 1

Probably a pars pro toto expression, from a very old root (related to Ancient Greek ), with the related meaning "ball".

Etymology 2

From (Land)Puller, a non-derogatory term for a policeman of the 18th century. In the 1960s, the term Bullen was first used by politically left protesters, possibly associating the American derogatory term pig of that time with the animal bull.