Are eggs sorted in specific order and what determines it?

Solution 1:

As far as I have experienced the only order of eggs is whether or not they're currently incubated (and their chronological order), since they're on top of the list. The rest is most likely random.
The (non-incubating egg's) order might even change when you pick another egg to incubate or get one to fill an open slot. This leaves only two possibilities:

• Random order
• Fixed order, but this means the setting determining the sorting order changes when you start a new incubation (contained Pokémon for example) ...which is very unlikely.
  This would mean, if they're sorted by ID the contained Pokémon changes every time the amount of eggs changes. Or the IV, or Level, etc. To be honest, that would be stupid programming since they'd be trying to randomize the order and could simply use a pure randomizing algorithm instead.