I just spent more than 20 minutes trying to knock down a frost dragon whose hitpoints were all gone...he didn't attack or anything once I took out his last few hitpoints, just kept circling dragon fang crater. I thought dragons were supposed to crash once you took them down to about 1/4 HP?

I finally gave up and just left the area.

I had the same thing but with a hawk. 2 different ones in fact. The first, on the way to the very first nord dungeon close to riverwood I killed a few hours into the game. I came back recently, saw the hawk fly around again, shot him with arrows about 5 times, he kept flying around.. With 5 arrows sticking around him, but NOT in him. Second hawk in riverwood, same thing happened. Must be a bug. I'll take screen shots if needed, it's kind of a funny sight actually.

That sounds like a bug, If you kill dragons too fast, sometimes they fly around really weird and very fast for a minute and then dive bomb into you.

Draugr Dragon?

It took me a while to whittle it down to zero HP...I'm a destruction mage. And once it was down to zero HP, it didn't fly "weird" or "Fast", it just stopped attacking and circled the area repeatedly in slightly different patterns. My Atronoch and follower still attacked it when it came in range, and I and they could hit it with attacks, but it just. wouldn't. die.

There has to be an area around that the dragon can land on. If you're on a mountain top for instance it won't be able to land anywhere in the cell.