Before the First Balkan War, Serbia and Bulgaria agreed on a particular division of Macedonia between the two countries. Serbia and Greece took much more territory than expected during that war, but Serbia was forced to give up all of their Albanian territory by the great powers during peace negotiations, such that an independent Albanian state could be formed - this had not been anticipated by the pre-war negotiations. This led Serbia to refuse to give up any part of Macedonia; if they did so they wouldn't have gained anything from the war. That was the initial point of contention. Clashes broke out between the Serbs / Greeks and the Bulgarians. The Serbs signed a secret defensive pact with the Greeks as a result of this - which the Bulgarian military staff suspected but had no proof of until the war had been declared. Meanwhile, Bulgaria was put into a similar situation with Romania over the northern territory of Dobruja.

The war started when Bulgaria's brash Tsar decided that it was worth the risk to try and take as great a part as possible of Serbian and Greek gains, while assuming that the Ottomans and Romanians would not attack, since the Russians had previously intimated that they would intervene to arbitrate a Serbian / Bulgarian split of the contested land exclusively (though they later denounced Bulgaria's efforts against their former Balkan League allies and started supporting Romania instead, which the Bulgarian Tsar somehow seemed to be unaware of.)

Essentially, circumstantial reasons made everyone have a reason to oppose Bulgaria, and Bulgaria have a reason to oppose everyone else. Then their overly-aggressive ruler misjudged the political atmosphere.