I used to get sent boards like that at epox.

The faliure is caused by a mosfet or cap dying. The burning caused by failed cap shorting (pulling loads of amps) or fet doing the same... or you were pulling ALLOT of amps from your CPU... but I doubt it on 12v supply.

As for the bending... like the other guys I would first be thinking that it was mounted wrong...
But it might be that the mosfets got so hot that the board bent... like a bimetal strip as one side was hot and the other cold... eapecially as thats right over the power plane where there are large areas of copper sheet.. not just traces.

I very much doubt that this is caused by heatsink/waterblock weight... or we would see it all the time.

Check all your standoffs to be safe. Dont want to do this again.