。
I ate because I was hungry.
Literal
Belly [subject-が] was-empty so ate [emphasis-んだ].
腹が空いてたから — casual contraction of 空いていたから. 食ったんだ uses the rough/masculine past of 食う (to eat, vulgar/casual) + explanatory んだ. Very casual male speech.
。
I ate because I was hungry.