I agree with Dean that some members have probably been complaining about commercial renters tieing up prime weeks/locations. We've definitely seen in the last year what I would consider a LOT of complaints that many reservations that used to be easy to get at 7 months or "last minute" are no longer available, so that could be adding to it.
My guess is that the inclusion of point renting in "WDW on the Cheap" kind of guidebooks has increased those wanting to get in on a great deal.
The continued acceptance and validity of doing business on the internet with strangers (not even corporations) is adding fuel to the fire.
And on a

note, Disney no doubt sees that while they increase resort prices every year, the price to rent points hasn't changed much in the five years I've looked at it.
So the "deal" that people get renting points grows into a better deal each year. Maybe it's something that is actually affecting resort cash reservations? Or least is perceived to be a contributor?