I have never had or seen this happen before. I was roaming in the wilderness of the rift. And a dragon just comes crashIng down! No warning at all. And his health was already halfway gone.

Random dragons are a normal part of the game. People actually complain about them being too frequent
Its health does seem weird, though. Maybe it got caught up in a fight with nearby animals? Giants, perhaps? This would explain why it was already injured when it attacked you.

Things do go on in this game without you.

The dragon probably spawned because you were in the cell, but got distracted by a Giant or a couple of Bears. They took out half his health, which is what triggers the dragon to crash land. Only they can crash land in a limited number of areas in the game. Think of them as Dragon Landing Strips. You just happened to be near the spot where it crashed.

When dragons fly around they attract a lot of attention. It's possible that he was fighting against a lot of enemies (such as above the fort where there's a crapload of imperials that will shoot at it as soon as they see it- they even land and whatnot, all off of your screen), so maybe you just didn't notice.

Horribly ninja'd

Normally that may be the case, but I have had an almost identical situation to the op. I came out of a dungeon and was walking along the road and a dead dragon dropped on me from out of nowhere. There was no monsters/creatures near by and it was an open space. It then disolved and I got a free soul for no effort.

I don't think that is the same issue. I have had that happen a few times and its a glitch. The OP's dragan was still alive and crash landed like a dragon does when he is hurt.