What was Alby so upset about at the end of Ch.36? Why?

Alby is upset because the doors to the Glade didn't close.... the Grievers can get in.

But Alby grabbed him by the shirt. "What happened? I'll tell ya what happened, shank. Too busy makin' lovey eyes to bother lookin' around? To bother noticing what freaking
time it is!"

Thomas looked at his watch, realizing with horror what he'd missed, knowing what Alby was about to say before he said it.

"The walls, you shuck. The Doors. They didn't close tonight."