He is just being a puppy. The fact that it is random is pretty much what puppies do. He decides one day "Ooh look! Toy!" and it is something he has been ignoring.
As an example, before retiring I had my younger dog in an outside kennel while I was a work. I put down 80 pound horse stall mats as the kennel floor. He would, on occasion, pick up one of those mats and toss it around (strong male German Shepherd.. the one in my avatar). I had to change things up since I feared him breaking a tooth! He did not do it every day, but I could not allow it at all.
