Origen was anathematized because some people believed he taught universal salvation, i.e. everybody, regardless of belief, will ultimately be reunited with god. Whether he taught that is debatable, but certainly much of his exegesis was quite abstract, which tracks with this argument that Origen likely didn't castrate himself: https://medievalhistorygeek.wordpress.com/2014/02/09/did-ori... Likewise, he was a polarizing figure during a formative time in the development of Christianity, so it's quite believable his opponents would spread gossip about him. In the first few centuries there were some virulently anti-sex (and anti-women) Christian cults that preached that any sex, including marital sex, was dirty and demanded things like lay continence or lauded castration.[1] Despite clerical continence being normative, mainstream Christianity forcefully rejected the notion that there was anything inherently wrong with (marital) sex, so to claim Origen castrated himself would be to insinuate his beliefs aligned with radical, non-orthodox, sexually deviant (in a sense) groups.

EDIT: [1] I originally used celibacy. But celibacy and continence are distinct. Celibacy implies continence, because only only married couples are permitted to have sex, but even married couples can be continent. Therein lies the confusion about the origins of modern-day Roman Catholic clerical celibacy and its relation to early clerical practices, including practices of the apostles.