The accumulation of oils and dirt from bathers is the biggest cause. Using a tile cleaner specifically designed for pools can clean it off. Household cleaners do not contain the needed balance of both oil/grease cutters and scale dissolving ingredients. In fact, these cleaners can actually dull a tile line due to abrasives or cause unsightly foaming. Even worse, they may react with the sanitizer, such as chlorine, in your pool.
Another helpful hint would be to regularly use an enzyme based product in your pool. These biodegradable products will control grease and oil before it builds up.