You either have a power imbalance or a failing PSU. Do some troubleshooting
to figure out which. Swap the mobo power with a GPU and see if it's still the PSU
powering the mobo that gets hot or the one you moved to power 5 GPUs.
My guess is you're probably using unpowered risers so the mobo is powering all the
PCIe slots creating a huge load on the PSU powering the mobo.
My risers are powered. And the hot PSU is actually the one that's not connected to MoBo but has 5 GPUs. I know it is highly inaccurate but HiveOS says that each of my gpu is generating around 65-70W and the whole rig around 600W. The rig was assembled in August last year and the winters were close so the temp. was not a big issue. I just recently started it again after 5 months during peak summers and fear this it might blow up or something. It is high quality Antec PSU though : HCG X1000, japanese capacitors and all that.