From Diablo Wiki
Sub-scenes is a term used by the D3 Team to describe the interchangeable scenarios and events that add to the randomization of Diablo III. They were originally called "adventures" or otherwise "side-quests", and then evolved during the beta to be called "events".
For instance, most levels in the game have one or more locations that can house a variety of events. In different games that same location might have a boss monster, an NPC with a quest, a scripted event, or nothing special at all. These different, random options are known as "sub-scenes."
Numerous of these sub-scenes are tagged as such in this wiki. More will be noted over time, but they can only be identified through experience. There's no way to tell a sub-scene the first time you see one, since you don't know if it's random, or there every game. Only with the experience of playing the same area multiple times can players recognize the randomized content within. Some players were able to run through the same areas several times at BlizzCon in 2008 and 2009, and spotted some of the randomized content.
The Origin of the Term
The concept of sub-scenes is an old one, since the D3 Team has talked about randomizing some of the puzzle-piece like sections of levels since the earliest Diablo 3 interviews. The actual term was first revealed during an October 2009 interview with Julian Love. 
- DS: I think Jay hinted during Blizzcon that there might be updated quests to keep higher level content exciting to keep people coming back and playing again.
- Julian Love: It's not so much actually high level versus low level. I think we have a plan that facilitates high level content. What I think you're really getting at there is replayability, and what that gets down to is the way we're exercising randomness within the game. We've put a lot of focus towards what kinds of randomness are really beneficial in supporting replayability and what things are just, heh, not all that important or not all that awesome. One of the things that we have in the system that we have is something that we call - well, we have a really technical name for it -- sub-scenes.
- What it comes down to is we can sort of like create a hole in the world that's like a socket that we can fill with just about anything that we want. It could be empty ground with a bunch of monsters on it, it could be a giant canyon that a really elaborate scripted event happens, it could turn into a caravan walking across the desert that you have an escort quest on. The point being is that this hole in the earth can randomly, at any time become something totally different. It could even be a doorway to a whole new dungeon that you've never seen before. So, the fact that the world is able to change things up and actually deliver different events, stories and quests is what's really going to make the game a lot more replayable at the both the low and high ends.