Derek was absolutely grounding Isaac after this; no more weird books and the kid could pick one damn job. Deciding against going around to the foot of the stairs, Derek didn't slow his run before grabbing the banister to pull himself up and vaulting up onto the stairs, using the wall to kick himself from going straight through.
There wasn't really much that made Derek want to look back, he could hear the zombies just as well as he could hear Rogers, which meant that neither of them were dead yet so there was still a slither of hope. Even if the stair case did feel abnormally steep, like it was a battle just to get up them.
It could've been the building, the way it was built, but Derek wasn't entirely convinced.