Because
DVC members, being a relatively small group in the grand scheme of things, are not bound by the typical rules of being more able to travel during non-school periods.
The single busiest time is probably the entire window from Thanksgiving through New Year's. The first couple of weeks of December is often slower at WDW as a whole, but the DVC resorts will be at max occupancy with many folks booking their trips 11 mos ahead of time TO THE DAY.
October and November are also busy due to the popularity of the Food & Wine Festival at Epcot.
Holiday periods (Spring Break, President's Day, summer months) are probably next with fairly moderate DVC crowds. And when I say "moderate", the resorts may still be close to max occupancy when you arrive, but it's much easier to get a room on short notice and/or book at a resort other than your Home.
Slowest periods typically include January, September and late-April to mid-May.