I suspect the connection to hot water is that histamines are being released.
During an 8 week bout with hives, I couldn't get to sleep because of the non-stop urge to itch. I was told to take a very hot shower before bed because it would cause my skin to release all its histamine, and give me a least a temporary break from the itching.
It did make the itching more intense for a short while, but then the relief comes.
There are connections between the release of histamine and the release of oxytocin.