I read an article in the Army Times that said most of the shortages in reloading supplies had actually begun by late 2005. This was because the Army's ammunition stockpiles had been depleted, resulting in them having to cancel some training operations to be sure of having adequate supplies for the soldiers in combat. They ordered something like an extra 4 billion rounds (IIR the number correctly) of ammunition above their normal demand for 2006. That was in addition to the 1 to 1.5 billion round annual capacity of the Lake City plant.
Additionally, competition with China had caused metals prices to climb significantly. Sierra commented that raw materials they had stockpiled and expected to last one to two years (because reloading sales had been in something of a slump up to that point), were depleted in just the first three months of 2006. That was both due to supplying military orders and due to civilian hoarder purchases. That meant they had to buy raw materials at high prices instead of waiting for a market dip, as they normally did. That caused the price of their bullets to shoot up, causing even more panic buying.
Same as with bullets, primer capacity was consumed. CCI decided to set up an additional line, but I've forgotten when the actually got it on line. Mid-2007, maybe. Then came the 2008 election, stimulating still more panic buying. That died down, but not 100%, as gun sales have stayed higher than before the 2008 election. Every time another public shooting incident occurs, there's another buying surge. The current one is probably the greatest I've seen because the threat of mindless laws being passed just so someone can say they've done "something", even if it's purely symbolic in effect.
Then, on top of all that, this article
says the Army is replenishing supplies yet again. So, a perfect storm.