“Miss Maudie’s tin roof quelled the flames. Roaring, the house collapsed; fire gushed everywhere, followed by a flurry of blankets from men on top of the adjacent houses, beating out the flames..”
In the confusion, someone drapes a blanket over Scout. When Atticus later asks her about it, she has no idea who put it over her. Jem realizes that Boo Radley put it on her. We can notice that Boo is always stay behind Jem and Scout.