It may be as simple as pure reservation service capacity, as the restaurants get more crowded, combined with free dining and/or
DDP.
Look at it this way. Suppose ADRs were just one day in advance, which they were at some point in the past. That means everyone who wants a reservation on Sep 4 needs to call on Sep 3. If only a hundred people are trying for restaurant X, they can probably handle that, both in terms of staffing and computers. But make the dining free, and now you have 5,000 people who all want reservations on Sep 4 for the exact same restaurant. Ignoring the staffing, this is going to kill the computers, because you're going to have dozens if not hundreds of people all try to get access to the same exact data. Extend it to two days out, and you'll still have 5,000 people, but half will want for the 4th and half for the 5th.
I hope that's clear. I'm trying to explain it using made-up numbers and without getting into the technical aspects of multiple simultaneous requests to a computer database. Or compare it to booking movie tickets. At least with AMC, when I check availability, I'll have seats locked up for about 5 or 10 minutes before deciding to either pay for them or cancel, and during that period, no one else can get those seats.