Usually this stems from using white or raw sugars in your wort. Sugar (or sucrose) contains fructose. Fructose is a complex sugar and hard for your yeast to break down, often in this process of breaking down this fructose your yeast will create unwanted esters (off flavours such as cidery, fruity or bitter flavours) that make your beer taste unpleasant. You can avoid this by using dextrose or brewing blends to ferment with your beer tin concentrate!