What event takes place as Perry’s convoy is backed up in a line of vehicles in Chapter 12?

While the convoy is backed up, Darcy gets out to urinate outside the Humvee and suddenly a bus ahead explodes; after the first explosion, a second erupts and Jonesy calls out "Daisy Chain!" and throws the Humvee into reverse. Darcy leaps into the fleeing Humvee as the unit tries to find safety.