It had to do with the worldships.

Those things were the size of the Death Star, could something that big break through the barrier?

This all totally reminds me of the Great Barrier that surrounds the galaxy in the Star Trek series. I remember one episode when the Enterprise tried to breach it and things go haywire after that.

The Vong obviously were forced to...reverse the polarities.

She can't take this abuse much longer Cap'n!!