Just to make this perfectly clear, while the government agencies are buying ammo in bulk amounts, this is a perfectly normal thing. They aren't receiving the large number of rounds all at once, it is a contract that spans over time so that they can guarantee a certain price, consistency, and a steady stream of munitions. Its simply being noticed more because of people panicking.
They also buy almost everything else in bulk, like weapons, food, toilet paper, medical supplies, etc.
The current shortage is because a large amount of people are scared about the potential bans and control laws. Hoarding and/or prepping is definitely a problem as well. New gun owners are likely a contributing factor here too.