My shower turns itself on ?

I have a Burlington, rainforest fixed head shower which has the hot and cold water pumped to it (not a power shower). The hot water comes from an Heatrae Sadia hot water cylinder and the cold water comes from a tank one floor up, both hot and cold run through the pump. As you would expect, it drips a bit when you turn it off but that is not the problem. After a few hours of it being turned off, it suddenly, for no reason turns itself on and water starts coming out of all the holes, but it doesn't initiate enough water to turn the pump on. I can stop it doing this for a few hours if I turn it on properly and then turn it off again. But only for it to drip like mad again a few hours later. There is too much water coming out for it to be just a matter of the pipes emptying. It's more like a build up of pressure...What's wrong?!!