Cassian slowly wakes up and checks his surroundings.
"Oh come on, another horror dream?" he says while checking the room filled with gore and corpses.
"Maybe next time" says he with dissatisfaction and goes to sleep.
As you might see, it's not his first time being woken up in a strange situation. The explanation is rather simple. He is a sleepwalker, an individual who has to go through certain scenario to wake up. Fortunately for him, Cassian can skip unwanted scenarios twice a day. However, the major flaw of that ability is the fact they his 3rd scenario will be forced, and there's nothing he can do.