My Labrador has never chewed up anything non-food related. (Except for when he was a young puppy, as puppies do)
I just moved house recently and whenever i go out i leave him inside as i don't trust the fences here (He's fine to go out into the yard when I'm home, but i get nervous about him trying to escape when i leave)
Anyway, I'm rambling, my point is that twice he has chewed up things while i was out (an hour or two max). Once a cheap fabric bag and the second time a small cardboard box.
He gets exercised and isn't left alone and bored for a long periods of time, if i had to guess i would say its stress or anxiety from moving house.
Its just shocked me a little because its quite out of character. I'd love some other opinions on what it could be and what i could do to help him.