Osira was about to head back and wait for her team along with her people at the very beginning of this, lets call it dungeon. This place, is far beyond her and her team's expectations so she feels a bit frozen and optionless. Because, what if there actually is something in there? What if it catches her off guard or at a blind spot? She could be dead before she knows it and let all of this contribute go to waste without ever having a taste of proper victory!
She grabbed a boulder blasted in because of the explosion, maneuvered it just a little bit so her back can meet the tunnel walls and was ready to wait. It would take at least an hour for her people to arrive here from the Cave they took residence at while the plan was in motion!
