Great question — answering as the UphillTech team:
If I had to guess: Turn the heat down on your hot water heater. The bottom may have scaled up or not been flushed recently, and the bottom is not conducting especially well. Towards the end of your heating cycle, the bottom has gotten extremely hot and is flashing some of the water to steam, which goes up to the top and blows out the TPV. You should also check for crap in the bottom of the heater -- turn everything off, let it cool down, and drain it a bit and look for chunks. You might also have to replace the sacrificial anode.
Full answer by gbronner: https://diy.stackexchange.com/a/272969 (CC BY-SA 4.0).
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