Alton Brown has this to say on it:
: I watched your show with Chocolate Lava Muffins, and wondered why you add the eggs one at a time. What could happen if you add all 4 eggs at once?
: The same reason that you drizzle oil into a salad dressing - because the beginning mixture contains fat and water, and eggs contain fat and water. We're basically making a large emulsion. Emulsions have a much better chance of forming if the elements are brought together slowly. If you don't add them slowly, you'll end up with clunks of eggs over here, clunks of chocolate over there - you won't have a homogeneous mixture. This is really, really, REALLY important, not only in this recipe, but in any batter or dough in which eggs are integrated after wet and dry ingredients are brought together.
Full Interview Here.