Ap holders at WDW can only hold 7 days worth of FP+ at any.one time, atand only 30 days in advance, unless they have an onsite resort stay. If they have an onsite resort stay, then they can book fp+ for the length of that resort stay at 60 days (this is my basic.understanding, I'm sure there are caveats i don't know)
Right, so you have AP holders snatching up all the FPs for the days which they intend to visit the parks. Remember that even though not every pass holder will visit every day, there is such a large population of passholders that on any given day they can make up huge portion of the crowds. With an estimated 1 million passholders even a small percentage of them showing up on the same day is enough to close down the parks.