Today it seems I fried my Asrock H110 Pro BTC+ mobo... Damage on the 12 connected GPUs I still have to assess

The connector of one of the 3 power supply cables to the mobo caught fire. Luckily I had the reflex to pull the power cord from the socket the moment I saw the flames... You can see the damage in this picture. The connector and part of the power cable have melted and the mobo is scorched.

https://www.dropbox.com/s/44e5scw4gkqxfwv/Foto%2015-03-18%2019%2022%2020.jpg
Before replacing the mobo, I want to know what went wrong. I don't want to die in a fire because my rig acts up at night.
The wiring scheme:

https://www.dropbox.com/s/vv8s1lhvuuvcojh/rig%20sata-molex.png
The red line indicates the wire that caught fire.
What happened:
- I shut down my rig using remote desktop
- After a minute, I went to check if it was really shut down in the other room. I wasn't, it was on fire.
What I think happened: the mobo shut down PSU1, together with the connected GPUs to that PSU. However, somehow PSU2 failed to shut down, still powering the GPUs.
The GPUs tried to draw power from the mobo, which was only supplied by the one power line from PSU2 which got overloaded and caught fire.
Any thoughts on this?