I am not 100% if it is an achievement, but you will find you often stumble across random events of that nature. I believe they are there to give the world a bit of random life to it as opposed to every single stone and tree being the exact same at all times. Sometimes you'll find a marchent and their bodyguards attempting to fight off bandits, other time you will find mages attempting to summon daedra beyond their control (as you mentioned).
They make for a lot of flavor and a lot of fun!