Look closely at where the front panel power switch connector is and make sure nothing is jumping the pins, check the back side to make sure nothing is shorting out the traces to turn it on. Try disconnecting the front panel power switch with the PSU off turn on the PSU and see if your board turns on by its self. I have the same board you do, and have had the same problem in the past with an old MSI board. If you don't see anything jumping the FP connector pins and nothing looks like it's crossed the traces on the back but it still turns on with the FP connector unplugged there's something wrong with the board. Also check to make sure the power switch in the upper right corner isn't sticking and nothing is crossing the traces on the back there as well.