そう考えるのも当然だ means “It’s natural for you/him/her/them to think so.”
の makes a verb a noun. In this sentence, そう考えるの is the noun.
そう考えるのは当然だ almost has the same meaning, but there’s little difference. そう考えるのは当然だ sounds like the listener doesn’t know or think that it’s natural for you/him/her/them to think so.