Very sad. It's hard to know what caused things to go the way they did, but dogs that wander around unsupervised in a pack together like that, especially if unaltered, can be a recipe for disaster. If one dog bites/attacks, the instincts of the others kick in to join their comrade in the fight. It's a very basic instinct, the same one that drives dogs to protect their owners when they perceive them as being under attack.