Hernia, Lyme diesease, hydrococele, varicocele, Hematococele (hematocele), Brucellosis, trauma (likely). Taking them apart with a couple of sterile gloves or taking it to your friendly large animal vet would have helped out. The spermatic cord could have been damaged and had venous flow cease. I wouldn't want to chance handling an animal that has Brucellosis. Undulant fever is a disease many vets suffered through in earlier years. This disease renders the Yellowstone bison inedible.
I've seen a bunch of deer that had edematous pouches around the neck and chest. Most folks thought it was a dreadful disease, but was simply from an isolated traumatic event they would eventually recover from. If this is the case with the testes here, I would think plain old fluid buildup would have been there. THe fluid being serum and not blood. Fluid could also be blood, pus or a combination.
