Soldiers taking out bandit camp?

Best guess: it's an unintended by-product of events triggered in an open world.

I came past the same camp (at least it sounds like it from your description), though on my way to, not from, Oxenfurt. I started fighting the bandits, who were "alone" when I arrived, when suddenly there were three more people around me. Took me a moment to realise they were guards/soldiers who weren't hostile. They "helped me" finish off the last bandit, but got too close and I must've accidentally (cough) grazed one who came too close to that last bandit. He shouted at me to sheathe my weapon, which I did, and they walked off, leaving me to loot the camp and bodies.

So I think that some overlap between fighting the bandits and the arrival of the soldiers is intended in the game (though the bandits weren't particularly difficult, and there were only three I believe), but for the soldiers to completely wipe out the bandits was probably not intended - after all, you want the experience for killing them. Again, I'm guessing here, but maybe you passed the area previously (speeding past on Roach), close enough to trigger the soldiers approaching the camp, but didn't stop to fight. The game has to handle certain proximity triggers, and, given the open world nature of the game, has to "activate" NPCs based on your proximity (obviously they can't all be actively doing things when you're far away). You've need a saved a game from a short time before you encountered this to test/confirm by riding there directly at full spead vs approaching slowly in a roundabout way and seeing what happens.