The ignition source appears in the photos to have been the circular component in the lower right portion of the second photo.
If I am not mistaken that was an MOV or TVP, and that is a classic failure mode for them.
Does anyone have photos of the interior of an un-burned supply for reference?
This is not meant to offer any excuses for the supply manufacturer and its failure or to place any blame, but out of curiousity what output voltage was the supply set to?
There are typically "crowbar" overvoltage protectors fitted on the outputs of switching supplies that may have triggered.
If it didn't then shut down the supply it would have had to absorb all the output power, and hence- up in flames.
I have seen this happen on machine tools that had older design switching power supplies.
Again, this is not an excuse for a cheap supply design. Just a question.