Typically when Bnei Yisroel travel in the desert they are preceded by the pillar of cloud during the day and the pillar of fire at night. Hashem Himself leads the way, and indicates when it is time to travel. When the Jewish People were crossing the Yam Suf, the cloud and fire followed behind them, separating them from the Egyptians. During this specific moment, Hashem was acting from din, from a place of judgement, as He waged war on the Egyptians. At the same time, Bnei Yisroel were stepping forward, bringing about the yeshua, the salvation, which moved us to then sing a shira. The shira welled up within the Jewish People, and they burst forth in song that was born through their initiative. - Library: Bnai Jacob Shaarei Zion - RabbiHauer.org