Learn French: Je suis bouleversé qu'elle ait triomphé hier. - I am deeply moved that she triumphed yesterday.

The main clause (Je suis bouleversé) is in the present tense, but the action of her triumphing happened in the past (hier). We use the subjonctif passé to show anteriority—meaning the action in the que clause was already completed before the emotion being felt in the main clause.

What exactly is triggering the subjunctive mood in this sentence?

The adjective bouleversé (deeply moved/overwhelmed). In French, any expression of strong personal emotion (like joy, sadness, fear, or shock) followed by que forces the verb in the dependent clause into the subjunctive.

How is the subjonctif passé formed in this sentence?

It is a compound tense, built very similarly to the passé composé. You take the present subjunctive of the auxiliary verb—in this case, avoir, which becomes ait for elle—and add the past participle of the main verb (triomphé).

I usually see bouleversé translated as "devastated" or "upset". Can it really be used for a positive event?

Yes! Bouleversé literally means "turned upside down." While it is frequently used for negative shocks (distressed, devastated), it is perfectly natural to use it for intense emotional reactions to positive events, meaning "deeply moved," "overwhelmed," or "profoundly touched."!