Too much water washes the soil on to the leach field where it blocks it up. The longer the water and soil sits the better it settles or floats depending on what it is. Only the water would get out into the leach field in a proper system unless you run too much water too fast. The thing to do is to run your shower water outside into it's own drain area, but it may not be allowed where you are. Used to be called gray water system. Look it up.