This pattern is used when you want to say that a result or situation is perfectly understandable, even expected, given the reason or context. It often follows a clause that states the reason or evidence, implying that the outcome is not surprising at all. For example, if someone studied all night, it's natural they are tired. The nuance is like saying 'It's only natural that...' or 'No wonder...'.

You attach も当然だ to the plain past tense of verbs (e.g., 疲れたも当然だ) or to the dictionary form (e.g., 失敗するも当然だ). With nouns, you attach directly to the noun (e.g., 子供も当然だ) because 当然 is a な-adjective-like noun. This construction is fairly formal and often appears in written language or careful speech. In casual conversation, you'd more likely say 〜のは当然だ or 〜するのも当然だ, but も当然だ is acceptable in moderate formality.

Be careful not to confuse this with 〜は当然だ, which simply states that something is natural without the nuance of 'even that' or 'also'. The も adds a sense of 'even considering that', reinforcing that the outcome is unsurprising. Also, note that も当然だ is not used for your own actions as a self-justification; it's more for commenting on others or general situations.