I purchased thousands of rounds of .22 last summer. Previously, I had never had more than 500 rds on hand. I presume many people are like minded, with many late to teh party.
People purchase what they can shoot. When other calibers became unavailable or unaffordable, people began buying what they could. The last ammo on the shelves was primarily bird shot and .22LR, now that is gone.
There are many SHTF and urban combat situations where I would opt for a .22 over all others. And if you can only afford $20 of ammo, you can shoot a lot more times with a .22LR.
I ordered some .22LR Super Colibri (no powder, just a small bullet that is propelled by the primer only) on Friday, just because it was cheap and available. By the time it arrived, the price at CTD had doubled (from 35 per 500 to 70)... and even the 35 was up from ~22 a month ago.