For me it was the part of the main quest where you're handing over your inventory to Delphine. At some point during the event, the game briefly immobilizes the player. Naturally, a dragon appears at that precise moment and prompts Delphine to go off and fight it, without triggering the part that lets me move again. So there I was, butt naked and stuck forever as Delphine ran off to try and fight the dragon on her own.

Oh the shame of it!

Actually to be serious I have found in my game that the area around Solitude to be a hotbed of dragon activity. I have had numerous Ancient dragon attacks both at the sawmill (Everytime I went there) and many dragon attacks within the city itself who scorched some important NPC's so to find out one would appear at that moment does not surprise me. Most of my random attacks there i subscribe to FT which I have cut back to nearly nonexistent and have not used it at all on Solstheim.

What you describe has happened to me more than once, OP.
These days I save at the inn.

Its a bit of a challenge when you have no armour, potions or enchantments, are a vampire and dont regenerate stats in daylight and so only have a bound bow to take down a dragon at level 50 or so. ><