If you haven't told it to retain variables, not sure why it's happening, although I know it sometimes does. However, if you don't want it to ever remember the selections that were previously selected (like if the user goes ahead a page and then comes back), then just create an On Show action to Reset Question with the target being either the question that's on the page (if there's just one) or All Questions on the Page (if there are more than one).
