It’s not a big mystery why people buy Bread, Milk, and Eggs… those are the three items (all pretty perishable, except for the eggs (which most people THINK are really perishable)) that you are likely to go through in decent quantity if you consume them at all and are stuck at home. If shoppers cannot buy said items for the next two or three days, they are essentially compressing three days for those high-volume perishable goods into just one.

(i.e.; if they usually do grocery shopping on Friday or Saturday, and are being told to stay off the roads, the Thursday, Friday, and Saturday people are all trying to buy those things on a single day. Since those items are all usually replenished once (or more) a day, it’s no surprise the store runs out.)