@CursedSilicon small filter cap. it's across a power rail and sits there to get rid of some amount of power line noise going to the chip right next to it. more likely than not, it failed short circuit, and because the power supply it was shorting is a giant ass ATX power supply that will happily dump several hundred watts into something, the capacitor ended up just exploding. once that happened, the short was cleared and the board was happy to continue running with just slightly dirtier power.