The thing is that I suspect that the problem is mostly not (yes, with some widely publicized exceptions) that you have people individually buying hundreds of rolls of toilet paper or dozens of eggs, but LOTS of people figuring that a bit of buffer stock in the house can't hurt because who knows what the future will bring.
I know in my case I did a couple shopping trips before the panic buying started. I bought nothing at all out of the ordinary but I did stock my house up a bit more than average on various things. You multiply that by the majority of people and it's not hard to clear out a store.
My pet theory adds that people were previously eating out a lot, and getting snacks from little shops. Now all of a sudden they are getting everything from supermarkets and cooking more often at home, so even without any hoarding, there's been a surge in the number of people eating home-made food at home.
I know in my case I did a couple shopping trips before the panic buying started. I bought nothing at all out of the ordinary but I did stock my house up a bit more than average on various things. You multiply that by the majority of people and it's not hard to clear out a store.