One of the intermediate stages of the switch-mode power supply (SMPS) use in a PC power supply most likely exceeded the voltage ratings of its components, which caused them to dramatically fail.
Depending on the topology of the SMPS design, the intermediate voltage may reach about 400 Volts, so if you accidentally switch the incoming AC mains voltage switch from 230-240 to 110-120 Volts (AC) (roughly multiplying from 2x to 4x), the intermediate stage could raise the voltage to over 800 V (240 x 4 rather than the intended 120 x 4), which if the components were only rated for 600 V maximum the components would fail (complete with sparks). Roughly speaking 600 V may be a reasonable safety margin for component selection, if there was little to no risk of having the wrong voltage switch setting (and it is fairly uncommon).
Now in newer PC power supplies (many/most in the past couple of years) are "universal" input, so they run from about 90-265 Volts AC (Vac) input and there is no more switch to accidentally flip. The requires more complex (intelligent) circuitry, but the complex circuitry also is more efficient, reducing overall power consumption of newer PCs with these newer power supplies.