Because you're always buying toilet paper in bulk, and everyone needs it.
Toilet paper is cheap and bulky, so stores only stock what they absolutely need. If a store is supplied once a day, it'll have something like two day's worth of toilet paper on its shelves.
Some incident happens, and people start to panic. By sheer coincidence one brand of toilet paper happens to be out of stock. The shelf space is huge, so as you walk past it you immediately notice it and think "geez, wouldn't it be awkward if that were to run out". You don't know how much you've got remaining at home so you grab a pack, just to be sure. Ten people do this, and because the packs are so massive another brand has just run out - making even more people consider picking up toilet paper.
You normally buy toilet paper maybe once every month, so if only 1 in 15 people pick up toilet paper during their panic shopping, those two day's worth of stock will quickly run out!
I've asked the same question. I think people are so pampered in western societies that the worst thing they can possibly imagine is not being able to wipe their ass with a fresh piece of toilet paper.
If you are old enough to have a conversation with someone who lived through the 1930s United States Great Depression, they had to resort to tearing pages out of the Sears catalog. After that, they would hoard toilet paper. And every generation since has done the same thing. But it started with the Great Depression.
