Likely a combination of several factors. Cost/hot weather/etc. I know where I work someone always seems to take their family to Disney in the Summer for the first time and returns to say it was so HOT/HUMID they will never do that again. It is exhausting trying to walk for miles in June/July/August Florida weather. As more people experience that or hear it from others, I think they decide to do Disney some other time of the year. Continually rising prices probably also causes people to look for other more economical places to vacation. Something is always down for refurbishment, so it is difficult to say how much that impacts attendance.