Have you ever known anyone with a hoarding problem? Hoarders often collect massive amounts of food and other items.
No, I don't know anyone like that!  What do they do with the stuff that they hoard?
Some hoarders believe that they will be hit by a natural disaster or civil unrest will cause essential items to be in short supply. 
That's a good reason, I guess.  Isn't there a tv show about hoarders?  I think I noticed it one day when looking for something to watch.
Yes! It's called Hoarders on A&E! The show explains how people begin to feel an extreme emotional attachment to their belongings and it becomes almost impossible for them to throw things away. Even if it's obvious trash.
Is it considered to be some sort of mental illness?  It sounds strange to me - I wouldn't have room to hoard anything!
houses may become a fire hazard (due to blocked exits and stacked papers) or a health hazard (due to vermin infestation, excreta and detritus from excessive pets, hoarded food and garbage or the risk of stacks of items collapsing on the occupants and blocking exit routes).. Hoarding affects more than just the person who has the strong attachment to possessions, as other people living in the home and neighbours can be affected by the clutter. Individuals with hoarding disorder have a quality of life as poor as those diagnosed with schizophrenia
) One theory suggests that human hoarding may be related to animal hoarding behavior, but substantial evidence is lacking.
Human hoarding.
Civil unrest or threat of natural disaster may lead people to hoard foodstuffs, water, gasoline and other essentials that they believe, rightly or wrongly, will soon be in short supply. Survivalists, also known as preppers, often stockpile large supplies of these items in anticipation of a large-scale disaster event.
Human hoarding Anxiety disorder and hoarding.
Individuals who meet diagnostic criteria for hoarding