Why it happens
Rice absorbs water as it cooks and its starch swells and softens, a process called gelatinisation. With too much water or too long a cooking time, the grains swell until they split and release starch, which makes the rice sticky and mushy. Stirring the rice while it cooks breaks the grains and releases even more starch. Some types of rice, such as short-grain and risotto rice, are naturally softer and stickier.