When a mouse has better housekeeping skills than you do....
If I had a small child I'd be showing them this to encourage them to tidy their toys.
Behaviourally, I'd guess it's down to their hoarding instinct. We used to keep unopened bags (very heavyweight plastic) of the dog's food in the garage till we discovered a hole chewed in it. Som time later we found a large pile of kibble in another part of the garage.