There is no conspiracy theory here. The federal government is expecting either a catastrophic financial collapse that could provoke nationwide food riots and all out civil unrest, another civil war, or even Armageddon. All in the very near future. Some theorize that the mass purchase of ammunition is an attempt to hoard as much as possible from the American public whom the feds believe may be in preparation for civil war right now.
High-level, reliable sources inside Homeland Security claim the agency is preparing for a massive civil war in America. The DHS source states that the federal government foresees and prepares for a massive civil revolt.
For more details, see this article
Let's hope that this is all some kind of bizarre mistake and that they really aren't planning for insurrection.