I'm no expert, but on my old system this happened too. It turns out the cold water pipe allowed hot water to travel back up to the CW tank as the pipe was an old imperial size which for some reason allowed this. Strangely it never happened on metric pipes. My father in law (who told me about this) slightly closed the tap on that pipe and it stopped. I can't explain how or why, but if there is a tap on that pipe, turn the tap down a bit and see if that helps.