Blizzard released a rather cryptic statement this morning concerning the offensive and defensive mechanics of Wintergrasp. Their entire statement is after the break, but it essentially comes down to the fact that there are some realms where one faction controls Wintergrasp nearly the entire time, and Blizzard has now shed some light on mechanics in place to correct this.
The faction hording of Wintergrasp happens particularly often on unbalanced realms. Because the faction imbalance can be so extreme in some cases, the smaller population faction has virtually no chance at winning back the Wintergrasp fortress and obtaining the heirlooms and PvP items, nor any chance at accessing the VoA bosses. This can cause frustration on the part of players in the smaller faction.
In this mechanic, which we presume is live, Wintergrasp zone mechanics will make it easier for the offense to gain access to siege vehicles if the opposing faction holds Wintergrasp more often. Blizzard is not specifying how long the opposing faction must defend Wintergrasp for the the offense to receive these significant buffs, but one can assume there is some sort of threshold.
The full Blizzard statement after the break.