He temporarily fell into a state of mental derangement.
Literal
He [topic-は] temporarily mentally [subj-が] derangement state [to-に] became.
精神が錯乱状態になる 'to fall into a state of mental confusion' uses the pattern [noun]が[noun]状態になる for describing someone entering a condition. 一時的に 'temporarily' uses the productive 的 suffix. 錯乱 'derangement/confusion' is a clinical/literary compound.