The soil and atmosphere of Buzzwole's homeworld is rich in naturally occurring anabolic steroids, to the point where even the trees grow muscles. By the standards of the Ultra Jungle, Buzzwole is not even a particularly strong pokemon; its frequent flexing displays likely originated in an often unsuccessful attempt to scare off predators.
By the standards of Earth, however, Buzzwole is a massively strong pokemon, with muscles species like Machamp could only dream of. Biochemical testing of Buzzwole revealed baseline testosterone levels well in excess of any known Earth pokemon, and above the median lethal dose for most of them. Moreover, Buzzwole is a fiercely competitive species, and every individual known to find its way to Earth soon partnered with a trainer and began to enter pokemon competitions; more often than not, it won them.
Many a trainer, embarrassed by their humiliating loss to a Buzzwole's fists, reported the Ultra Beast who defeated them to local Pokemon League authorities for the use of performance-enhancing drugs. While the allowable biochemistry of competitive pokemon varies from species to species, many still condemned Buzzwole for testosterone levels which greatly exceed all of them. Many elite trainers, including a couple dissident voices on the rules committee, call for a species-agnostic limit that would effectively ban these pokemon. Steroids chants, once the bane of suspiciously strong (or previously suspended) fighting pokemon, now pepper arenas whenever a local hero's rival summons a Buzzwole, even one physically weak by the standards of the species.
For now, however, trainers have resorted to using Flying, Ghost, and Psychic pokemon to keep the powerful Buzzwole in check. And in fairness to Buzzwole, the metagame of high-level pokemon tournaments has always shifted with the discovery of new pokemon – whether they came from far-flung regions of Earth or beyond the stars!