It might not be enough to stop the Grundfos pump. It needs to be valved off upstream. Also, do you have roof-mounted solar? An expansion tank on another hot water heater?
Your street valve (meter box) might also be defective.
If you have an outside irrigation system, with an open valve, it could backfeed into your home plumbing as the daytime temperature increases, thus increasing pressure in the irrigation tubing or hose.