Aizawa wanted to close the dorms because of the villain attack. The villains were everywhere and every student should be closed in the dormitory.

But to he's surprise, when he wanted to close the dorms, you weren't there. He started to searching for you and found you in the backyard. He came closer to you

Aizawa: Why aren't you in the dormitory..? You know it's dangerous here. We let you all out when the villains will be gone.