I'm really sorry you're dealing with this, it sounds awful, and I'm glad the injuries were fairly superficial!
Unfortunately the chances of figuring out what exactly triggered the fight are pretty slim, unless it happens again while you're watching. Maybe the older dog has some sore joints and the younger dog stepped on him in a way that really hurt. Maybe the younger dog is being more of a jerk now that he's reaching sexual maturity and the older dog had enough (we've had a couple altercations between my boys now that my youngest is old enough to start asserting himself more, though usually over high-value food or chew items). Maybe there was an upsetting sound or smell from outside (or a mouse or something inside?) that got them worked up.
Splitting them up when you can't supervise is a good idea for a while, just to prevent any escalation if there is rude or inappropriate behavior happening when you're not watching. I'd also want the vet to check both dogs over for hidden issues like joint/muscle pain or anything that might be physically stressing them. I hope it was just a fluke one-off occurrence and things settle down for everyone.
Unfortunately the chances of figuring out what exactly triggered the fight are pretty slim, unless it happens again while you're watching. Maybe the older dog has some sore joints and the younger dog stepped on him in a way that really hurt. Maybe the younger dog is being more of a jerk now that he's reaching sexual maturity and the older dog had enough (we've had a couple altercations between my boys now that my youngest is old enough to start asserting himself more, though usually over high-value food or chew items). Maybe there was an upsetting sound or smell from outside (or a mouse or something inside?) that got them worked up.
Splitting them up when you can't supervise is a good idea for a while, just to prevent any escalation if there is rude or inappropriate behavior happening when you're not watching. I'd also want the vet to check both dogs over for hidden issues like joint/muscle pain or anything that might be physically stressing them. I hope it was just a fluke one-off occurrence and things settle down for everyone.