An unknown future. No one knows exactly what changes may, or may not come as legislation is debated. Several proposals would include huge taxes on the sale of ammo. People are stocking up on 22 and other ammo in anticipation of large taxes in the future. I'm not anticipating a major problem, but have been wrong before.
Another reason is lots of first time buyers. I'm a long time gun owner and have enough ammo on hand for several years if necessary. But I built up that stockpile gradually and have had it for years. I try to maintain a certain level of ammo on hand and usually buy replacement ammo as I shoot it up, but usually in bulk and only at good prices. I've not bought any new ammo in months and don't plan to until things settle down. But there are lots of guys who had only a few rounds available for each gun, and lots of new gun owners trying to build up supplies to get to the level where I am now.
Got a neighbor who was never interested in an AR before the Newtown shooting. Since then he has bought 2, along with around 2000 rounds of ammo and about a dozen 30 round mags. He paid a small fortune for them.