In Chapters 17-18, why do the boys have to flee from the stable where they'd been living?

In Chapter Seventeen, the boys were staying in the stables, and many people joined them there. The Jackboots eventually arrived and took Misha and the others to the ghetto. Uri was not there, but he joins Misha in the ghetto in Chapter Eighteen. He does not tell Misha where he's been...... he only tells him that the people who were sleeping under the newspapers are no longer alive.

Why did somebody rat out the boys for sleeping in the stable