I don't know of any particular Russian antecedents, and the Russians definitely didn't have much familiarity with eunuchs within their own culture or from other people's cultures (castrati travelling to Russia were pretty rare, and their consumption of opera was low compared to other countries, but that's another story...) However, Christian cult castration (or accusations of it) is not unique to the Skoptsy at all. Perhaps as old as Christianity... if you have access to JStor or are in the Register and Read program, you should check this article out. If you're in a situation in which sexual restraint or chastity is highly valued, certain body parts start to seem a liability I suppose. There are other Christian expressions of chastity that don't move on to castration, but it is a mental jump that historically happened more than once. The only really unique thing about the Skoptsy is their relatively large number of members, and their becoming a bit of a cultural touch-stone for castration.

I feel like I should warn you - I have done very little reading on the Skoptsy, because I don't like religious self-castration, but also because a lot of the historic sources on the Skoptsy I feel unable to ethically to work with, because the last of them were medically studied against their consent by the German military. Not to say historians of atrocities can't or don't ethically work with these sorts of records because they do, I just find them too upsetting myself. So if you want to look into them for your own research, just be aware of that.