There are a few different reasons that dogs show destructive behavior: most often it is because they are bored when left alone, but it can also be due to past reinforcement (ex. always finding yummy food in the trash), teething in young dogs, a natural inclination to chew (this is a very natural dog behavior), or separation related anxiety. Separation anxiety destructiveness usually is directed at doors or windows, or items near them when confined to a crate. If you think this is the case, we would recommend consulting with a veterinary behaviorist. If the dog is always destroying the same item or items in the same spot, we may need to block his access to that spot when you are away since he is likely finding something reinforcing there.
Otherwise, it is important to restrict their access to items that you do not want him chewing so he does not start to practice this behavior and make it a reinforcing habit. If he does get a hold of something, trade him for something more valuable that is appropriate for him to chew on (a toy, a bone, etc.). This is your dog's way of communicating to you that they need more to do physically and mentally - you may need to take your pup on extra walks, have extra play time, or have extra mental activity.
Most of the time destructive behaviors can be solved with a combination of crate training, higher levels of enrichment and exercise, and providing appropriate things to chew on. See the links for suggestions on increasing exercise and managing this behavior. If you have already tried these things without success or think that your dog has separation-related anxiety, please email email@example.com .