bombshell announcement; shocking declaration
Used for a sudden, surprising, or explosive public statement that causes a stir, often in politics, entertainment, or personal news.
His sudden retirement was a bombshell announcement.
A bombshell declaration came out at the press conference.
衝撃発言 is a more general 'shocking remark', while 爆弾宣言 emphasizes the explosive, public, and often formal nature of the announcement.
Compound of 爆弾 (bomb) and 宣言 (declaration), literally 'bomb declaration'. The metaphorical use of 爆弾 for something shocking is common in Japanese.