Drop Dead Dragon

Solution 1:

I did some looking and there appear to be a good variety of glitches in which dragons can die. Some of them might fit your scenario:

1. Dragon flies (glitches) into ground and dies. Thunder sound, death throes animation, dead. No soul absorbed but this guy was pretty far away and never engaged the dragon:

2. There are a few recorded accounts of dragons dying when they land. This one shows a dragon that was at full HP who lands, dies, and the soul is still absorbed:

3. Same case as before, but the dragon was damaged:

There are plenty of other videos in which dragons die as the result of glitches, but I think that these three best match your scenario out of all of them. Let me know if this helps!

Here is an example of a dragon simply spawning dead in front of a player; the "dead dragon glitch" you mentioned earlier. This one doesn't match your scenario too well, but appears to be fairly common:

For the sake of showing that it appears to be possible, here's a video showing two dragons going at it (uhh... fighting):

Solution 2:

If its a black dragon then it is Alduin. It has something to do with him (or it?) roaming around Skyrim trying to resurrect all dragons from the grave. And if anything (dragon, human, deadra etc) hit him(?), that thing will become history in seconds...

...except for you, Dragonborn of course.

BTW nice poem.

Solution 3:

This has happened to me before. Was confusing as hell, took me a bit to figure out, but it's definitely story-related and definitely worthy of a poem.

It's Alduin. Not sure why, but sometimes he turns on other dragons and pretty much one shots them. Maybe the first dragon was flying by, breathing fire and it hit Alduin, causing him to turn hostile.