I wouldn't be surprised if this were a temporary change due to the economy and Disney installing their all-encompassing new reservation system.
This is the only rational explanation. Why else make the CMs receiving ADR requests take 3x to 6x the number of calls when everyone's 90 days hit, when you could have spread that # out over the previous three months? No reason, except the new system and the economy.
I have no doubt they wanted to keep a closer watch on reservations made so as to know which promotions to roll out; in addition, if anything goes wrong with the new system, botching up 90 days worth of reservations is much better than 180.
When the economy picks up, and the phone and online rez systems are operating without (much) failure, and the CMs at the call centers are brought up to speed, and people begin booking their hotels months in advance again, I believe the policy will return to 180 days.