Killed a dragon with Whirlwind Sprint?

You must have triggered a glitch or bug, which may have seen you moving through the dragon as an attack, and not a movement technique. If you can't preform it more than once, it was most likely just a hiccup in the game. A damn good one, if I may add. :3

I know this question is a bit old, but I experienced something similar on a few occassions.

It is possible that a dragon actually crash lands (taking heavy damage) and dies, which might have been the cause. In my experience there are two ways this can happen:

• Dragon Rend enforces the dragon to land, if there is little space to land, it can take time for the dragon to find a spot and may actually be too late. Long story short, the dragon is forced to the ground (and takes heavy damage) and if you are in a snowy area, it will leave a sliding trail.
• The dragon tried to land (potentially after taking damage) but 'glitches' into a tree/rock/bush, this may trigger the dragon crashing into the ground, but you will see little of this because it is already close to the ground. However, it may still die!

The second option would seem viable and would also explain why you would be able to sprint through it (imagine a very small spot between the dragon and the ground that you passed through). Not receiving the dragon soul might be explained by not having attacked it, but the dragon dying none the less.