Anything.

Could be an unfortunate freak accident that disables the electricity grid.

Could be that all trees around your home all fall down and block the roads so you can’t get to the store.

Could be that you break your leg and can’t go to the store to get food or water for 10 days.

Could be flooding, could be drought, could be wildfires, could be a reversal of earths magnetic field or a solar storm that destroys all electronics and throws us back to the stone age.

3-10 days is the realm of “something could happen that meaningfully disrupts the general supply chain”. and also still reasonable to expect of people. Like, they’re not going to ask you to stock up 6 months worth of water and food in a 50m² apartment. That’s not going to happen. Too expensive, don’t have the room. And they’re also not going to ask you to stock one (1) cookie for an emergency in case you get hungry tomorrow afternoon.

And also, if the problem is so huge that the government, military and NGO people can’t fix it in 10 days, that’s a pretty damn big problem and it could be that stocking up from 1 month doesn’t help much more than 10 days, because they can only solve it in 5 years.

3-10 days also buys time to figure out evacuation without having to think about literally the next meal.