The second Doom3 novel "Maelstrom" summed it up pretty well, in my opinion. From the book, during the second wave of demonic energy:
Uri could hear their yells, the screams, the few barked words of confusion, trying to marshal each other's brains to understand and, Christ, do something.
But with the air came the noise, and having seen what happened last time, Uri pressed tight against the wall, neatly into a corner, and waited for the inevitable, waited for what came next.
*Skip a few pages*
Uri wasn't the main character, the event was just told from his perspective. It kind of makes sense, in light of the supernatural forces at work in the story, that to avoid becoming a demonic undead, you would have to hold on to some shred of your humanity with all your strength. It's a reasonable explanation as to why some people were able to remain human.
I have a name, he thought. Even as a roaring train of sound and light rocketed past him, almost ripping him away from his crouching position at the wall, he kept thinking, I have a name. I'm, I'm...
And he knew that if he couldn't recall his name now, and who he was, that something would begin to slip, that whatever still roared past him would somehow be able to touch him, capture him... change him.
So he absolutely knew he had to fight to keep coming back to that point. My name, my life... I had it just a few seconds ago. It was there, sweet God, it was just there. Please don't let it just vanish.
But then the roaring noise seemed to ebb, and with it, the light, the color, began to fade. The roaring express moved on, seeking other corners, other people. And then, there it was:
I'm Uri. And I'm okay.