There is no authentic explanation about the relation between Rendaku, a phenomenon in which tho initial voiceless sound of tho second member in Sino-Japanese compounds tends to turn voiced, and the accent.
The conclusion drawn from my study is as follows:
1) Rendaku occurred most frequently when a departing tone (去声) was preceded by it.
2) Rendaku occurred most slightly when the accent of Rendakuji (連濁字) was a departing tone. This was because the accent of the second member of the compound changed from a departing tone into a rising tone (上声).