You can't easily put a 8 pin PCIe power cable into a 8 pin EPS 12V MB connector, they are keyed differently.
Both of the cables contain only +12V and Ground lines, due to some really unfortunate decision making by the PCI-SIG, the pin assignments are opposite each other which means that you were driving +12V into the motherboards ground; see the image below.
An ATX power supply is required to provide short circuit protection:
An output short circuit is defined as any output impedance of less
than 0.1 ohms. The power supply shall shut down and latch off for
shorting the +3.3 VDC, +5 VDC, or
+12 VDC rails to return or any other rail
The voltage regulator circuit (aka voltage regulator module) on your motherboard took the 'brunt' of the reversed polarity on the ground plane. How much protection that circuit has from an event like this varies from model to model - there is no standard. The good news is that components in this circuit are easy to troubleshoot with home electrical equipment (multimeter), and can be repaired. The bad news is that if you can't do the repair yourself (need solid soldering skills), the cost to get it repaired will be around the same price as a new motherboard.
See this link for additional info on a motherboards voltage regulator circuit