Why does Montag hide his book collection in the backyard?

Montag his the books in the backyard because Mildred was burning them one at a time.

He searched the house and found the books where Mildred had stacked them behind the refrigerator. Some were missing and he knew that she had started on her own slow process of dispersing the dynamite in her house, stick by stick. But he was not
angry now, only exhausted and bewildered with himself. He carried the books into the backyard and hid them in the bushes near the alley fence. For tonight only, he thought, in case she decides to do any more burning.