It's the restoration of the originally planned Balkan War division of Macedonia - the problem arose after Austria threw a hissy fit over what's now Albania and basically threatened total war if Serbia didn't retreat. Meanwhile primarily Serbian troops were occupying Macedonia. So, the Serbian government wanted to renegotiate the division of Macedonia to compensate for the unexpected loss of Albania and the resulting landlock. But then the Bulgarians got really pissed. So the thing was taken to the Russian Emperor to decide. But instead of waiting for the final decision, the Bulgarians attack, and trigger the Second Balkan war.Why exactly is Bulgaria gaining territory? Were they expecting Bulgaria to be on the Allied side?
Also, why is Serbia losing land to the south if the Allies win?
Anyways, there's no way that scenario would unfold - a lot of Serbian generals wanted to basically raze Sofia to the ground and annex it and then send the rest of Bulgaria back to the stone age over their actions in southeast Serbia, and Albania was officially a neutral state.