I randomly got all of the DLC for Fallout New Vegas (I only bought the original retail version back in 2010, that installed via steam).
So Bethesda has done things like this before. My friend's and I theory is the internal code for the Ultimate Edition and the regular edition are the same, in terms of Fall Out New Vegas, because we both got the DLC around the time Ultimate Edition came out.
Perhaps it was linked to the 'Legendary Edition' coming out for Skyrim? I have no idea. Point is, something similar has happened to a friend of mine, and myself concerning other Bethesda games.