Fortified wall, called by Death Runners 'New Berlin's Wall', separates the wasteland and ruins of the outskirt from the central part of the city under control of the mutants. Not exactly a place for holidays. That's UAV's final stop, if it's not destroyed earlier by some rogue mutant packs. Once unloaded, most UAVs end up in the moss surrounding the wall or are being used for augmenting the wall itself.
Sometimes things just go bad. There is no shame in running your ass off, when you're about to get fried if a gang of Berserkers won't get you first.