I believe it is two fold:
1. The demand for ammunition of all calibers is straining raw material supplies and manufacturing facilities;
2. People are shooting .22lr much more due to the increase in shooter numbers, and the affordability of .22lr as compared to centerfire. This increase results in people purchasing more for current and future use.
I do not believe the hoarding argument. Between the military, other departments within the government, and increased shooting interest we are taxing our supply lines. Add to that that people will purchase to meet their needs and replenish their own empty shelves whenever they see something, and you a a perfect storm of supply and demand.
I was told last year that I was hoarding, because I had over 5,000 rounds of .22 lr in two different varieties. Guess what: I could not complete my league shooting this winter, because I ran out of quality .22lr for bullseye and couldn't find more. The only .22lr I have right now is set aside for Firearm Safety and Hunter Education class this summer. Pretty hard to run a class with our ammunition!