Evolution of が (ga) particle: How did use as conjunctive arise from use as a subject marker As mentioned in this post, > started as an attributive case particle, became a subject particle, and then turned into a conjunctive particle. and in modern Japanese it has its main uses as both a conjunctive particle and a subject marker. While the answer in that question covers its evolution from an attributive particle to a subject particle, how did the use as a conjunctive particle arise? If anything, I would expect "wa" to have evolved as the conjunctive particle instead of "ga" since it already plays a role in contrastive nuance.

I would direct you to 'A History of the Japanese Language' (Frellesvig, 2010, p245) to see an explanation with examples. I must admit that his explanation is rather technical, but I have tried to paraphrase below.

According to the above source, the usage of as a conjunctional particle emerged in Early Middle Japanese (EMJ). As you mention, had previously been used as a subject marker. This subject marker function evolved to apply to cases in which a sentence had two clauses, one of which was a "headless nominalized clause" (a type of relative clause). Where would previously have marked a noun head as the subject, it began to mark these headless clauses. Frellesvig claims that we can interpret the usage in two ways: (a) as functioning to mark a type of relative clause, or (b) as joining two coordinate clauses. It is (b) which we recognize as the conjunctive marker function that we see in Modern Japanese.