Well, there's fan speculation that they're a hive-mind separated from their Queen, either by mistake or on purpose. Or just scum: a prison transport or somesuch. It doesn't take much for modern humans to sink into open sewers and tin shacks, technology or not.
Last film here: National Treasure 2. Almost delightfully stupid, but, in the end, too stupid to delight. The first half is more Fortean trope-spotting fun.