you sure you got no mods? sounds like wars of skyrim
After an update, Spawns can multiply for the first few game days. It has to do with the game putting a spawn in place, then after the patch, adding more spawns to the same place without removing the ones already there. Back in the Beta when we were getting a patch update each day, or near to it, I had a few battles with multiple enemies. Not with two dragons at once, but I did have back to back dragons. I think the most in one battle was about 10 enemies.
Things like this settle down after a few game days. So, either enjoy them while they happen, or wait somewhere for 73 hours to get normal spawns again.