Anything shorting something on the motherboard ?(motherboard mounting studs, perhaps a stud where there shouldn't be one?, slot covers, IO back plate etc). At this point I would test the board outside of the case (on the motherboard box or something non conducting and non static) with a bare minimum of components and wires needed to actually run the system (CPU&cooler, a GPU, only one RAM stick, power but no case headers). And if you have the option, try with a different GPU, try GPU in a different PCIe slot, swap to the other RAM module and so on.
Also, while out, inspect the board both sides for any bent header pins, bulging/leaking caps, missing SMD components, burnt spots, component legs not snipped close to the PCB, CPU socket and CPU pins and so on.