I'd have to say the most likely explanation is... that the tank was structurally weakened during the Cylon nuclear detonation... during the first attack, and that caused it to buckle and rupture.
もっとも考えられそうな原因は・・・ サイロンの攻撃で受けた放射線が タンクの外壁を傷め・・・ それが撓んでタンクを破損させたかも知れません