I've got a 2 year old mixing valve in the upstairs tub/shower. For the last few months, the tub faucet starts to run a light flow of cold water whenever the washing machine downstairs is filling. There are no other leaks, and no other aspects of the household plumbing are acting up. When the washer is not filling, the tub/shower works normally, with no leaks, dripping, or other oddness.

I suppose the obvious solution is to replace the core/stem of the shower valve, but I'm curious about what could cause this particular problem. If the shower valve usually shuts off normally, why would it run when the washer is on? Is the stem replacement the right fix?

Thanks,
Larry