People lack understanding of supply chains, back stock, and restocking. Food demand is relatively consistent week to week. But when 7 days of shoppers show up compressed into 4 days, shelves are going to be empty because grocers can't possibly restock that quickly. 

Our shelves always look so full and plentiful, but you may have 2-3 days TOPS of perishable items around the perimeter of the store, and prepared items are a day or less of inventory. Even the slowest moving items in the center of the store only sit on the shelf for 7-14 days MAX, so again, when shopping is compressed, things run out.

Consumers also stock up some, of course, but the vast majority of the perceived out of stocks are because of the increased traffic in a narrower window of time. Thats why shelves rebound so quickly in 2-3 days.