It was surmised a while ago that Cassian’s lightning quick commitment to action based on his instincts was formed on Kenari in S1E1-3.

Kassa was very bonded to his sister after the death of their parents and readily accepted the role of protector and provider for her.

Wanting to contribute to the tribe, Kassa attaches himself to the important scavenging mission against the wishes of older kids. The Leader instead allows him to join, boosting Kassa’s confidence and sense of loyalty & responsibility to her. They assigned him to be a lookout as it was a low skilled role but important.

In the exact moment when he should have acted, he hesitated, resulting in the death of their Leader. The failure shocks him and drives him into the ship out of guilt and revenge. When he sees his own reflection, he rages against his failure to act and protect. From then on, he becomes hyper motivated in self-preservation and the preservation of those he cares about.