Hmm, found a few random encounters before doing the quest I think you mean...
So I think random spawns happen even before that particular quest and moreso as you level up or progress along the story.
Right after you're sent for the Horn of something or other, you're led to a dragon grave where a dragon I presume is the big boss resurrects another dragon? I'd downed a few by that point, had more souls than words and took out the resurrected dragon easy as pie 

So I think random spawns happen even before that particular quest and moreso as you level up or progress along the story.