Fire and destruction. Sundering the grail may have put an end to its mockery of justice, but it's unleashed the filth within to cause its own havoc. There's something more as well, a reverberation in the air, a feeling that is at once alien and familiar. An instability, an anomaly, an uncertainty that runs as deep as the world itself. Something terribly wrong, and terribly dangerous. If Kiritsugu wants to survive, he needs to run, now.
Given what he's seen of this place, that isn't a bad hypothesis.
He's spent a while exploring the Pines. It might be reasonable to try and sleep soon. Even if he decides against sleeping, if he's not back in the cabin by the time Bran wakes up, he might be worried about where Kiritsugu's gone, and certainly Thia and Ira will if he's still not there when they're awake.
He's getting tired. He should sleep.
He passes out again, hoping that this time that will mean no dreams.