First, whatever caused them to flee the tent happened suddenly. They had no time to dress properly for the cold. Whatever happened was completely unexpected. They might have seen and heard a missile explode, but had no idea toxic fumes would momentarily fill the tent.
Second, it was something they smelled, tasted, heard or felt. It was pitch black, and other than a bright flash of light, they couldn't have seen much. Maybe they felt the ground move beneath them, maybe they smelled toxic fumes, maybe they heard something and thought it was an avalanche, maybe they heard assailants yelling at them to step outside, etc.
Third, each one of them believed the danger was real. If some of them felt otherwise, why not remain in the tent (unless mass hysteria happened)?
Fourth, each felt they had a better chance to survive by reaching the trees. Whatever the reason, they felt the trees would be safer compared to their exposed and unprotected spot on the slope. Unless, of course, they were ordered or forced to walk to the trees.
Just my thoughts, feel free to add or remove assumptions from this list. Or create your own list.