I wouldn't say they did so to gain more territory, moreso to keep their relatively recent acquisitions. They had a Serbian nationalist problem in Bosnia which they'd acquired from the Ottomans 35 years prior and so their attack on Serbia was one part face saving punitive war to avenge the slain heir and another part stamp out the perceived ongoing threat to their integrity.

But yeah, it was very much a case of punching a brick and the wall falls down upon you.