Does the si passivante come from the Latin passive?
There is a well-attested grammaticalisation path going:
Relexive > middle > anticausative > passive > impersonal
This means that, in the contest of grammaticalisation, there are many occurrences of reflexive constructions turning into passives. Grammaticalisation itself is a very broad topic, but it probably suffices to say that it's a form of linguistic change making words or constructions more "functional" at each successive step; a simple example would be Latin MICA 'breadcrumb' > It. mica, a negation adverb void of any lexical meaning and thus purely functional. In the case of the si-passive, we can hypothesise a similar line of development starting with the Latin reflexive.