Sometimes NPC's can kill Dragons.
I've had an "incident" near the top of Throat of The World where I was the only person, fighting a dragon. Mid-flight the dragon just fell, still alive and breathing fire. It hit the floor about halfway down the mountain. Climbing down to kill it, I found out the Dragon had actually FALLEN to its death.
Yes that's right. The Winged Beast had FALLEN out of the sky for no particular reason! It wasn't as though I had damaged it enough for this to happen, the fight had only just started and I'm playing on Master with several mods that increase difficulty.
I think it's just a glitch or something. Just be glad, it's a free Dragon Soul
