Definitely do not attempt to unsolder that row of pins to separate the two boards. If you are a beginner at this, you are very likely to damage the PCB traces while doing that. Which then eliminates the option to return it to seller as faulty.
It could have been just dust that arced when you disturbed it, though from your photo it doesn't appear dusty. I had a PC power supply that blew up due to dust accumulation only.
Another possibility is tin whiskers. Those heatsinks look like they are tin plated, and maybe have some tin surface roughness developing. It's an obscure problem, google 'tin whiskers'. It has been discussed a few times here. Tin plated metal surfaces tend over time to grow ultra-fine long whiskers of pure tin. You might have blown one of them loose.
Or, you were just super unlucky and blew an old iron filing or something into the wrong spot.
If you remember where the flash seemed to be, closely inspect that area with a magnifying glass. There should be some evidence of what went phut. Cracked component, bright arc-points on metal, dark flash-metalization on nearby surfaces.