I had something like this happen to me for the first time just last night, thought it was strange in all these months I've been playing that this was the first time, so I googled it since none of my friends seem to have had a similar experience.
Really wish I had one of those video capture cards for my 360 cause it was an epic fight. I was doing 'The Cursed Tribe' daedric quest and I fast travelled back to Largashbur, and as soon as I load up they're fighting a giant as usual. Then outta nowhere 2 ancient dragons swoop down, I use my call Odahving shout, and then a blood dragon shows up.
Of course all the Orc's there died, and it was disappointing to have to reload a previous save. Reloaded my autosave from when I was in Riften and fast travelled to the stables just outside instead. The 2 ancient dragons attacked me there, and when I got back to the Orc's no dragons popped up lol.