Hoarders seem to keep things that even seem like trash. Like stacks of 20 years worth of newspapers.
Yes, and it can be disgusting, but more importantly a fire hazard or health hazard for them!
Why do you think people develop that behavior?
I think it's something that develops in their personality psychology, but can be for many reasons such as being lonely.
I understand. So hoarders may have other issues at play.
I have no doubt.  Think about animals, like squirrels, that hoard their food.  They do this to survive.  It's quite possible that humans hoard for the same reason.  They feel they have to in order to survive.
Wow, I never thought about it that way. So interesting.
Other things can lead to people becoming hoarders as well, like civil unrest or natural disater.