Here in New England, it's because of one blizzard people still remember (I didn't live here yet) when those items ran short.
I also think it's because they're what I call the "mid-week items". Many people do their main grocery shopping once a week, but stop quickly for another gallon (or half) of milk and a second loaf of bread in the middle of the week. The don't like to buy it all at once because it's better fresh. So it's that "little" stop people would normally be making on the way home from work that they are trying to squeeze in before bad weather.
That would be me, I do big grocery shopping but do end up at the store during the week for another couple gallons of milk and loaves of bread so if I know a storm is coming I just go grab that stuff. I don't drive in the snow, even an inch so I go before it comes.
Last year we had a horrible ice storm and were without power for days, some people over a week. We weren't expecting it to be so bad but it didn't matter because people couldn't even go to the stores, they were all closed for days. After that I have no problem being one of those crazy people stocking up even if they are just calling for a few inches.