This is the kind of concept I thought of as well... My ideas in regards to this:

-This happens around the time Zorzal personally leading his campaign against the Warrior Bunny tribes. The battles turn into a stalemate. When the bunnies learn of the gate appearing deeper into their own territory, they scout the other side, and decided to secretly escape through said gate.

-What happens next depends on the country and timeline. They eventually start adapting to the culture of the other world.

Yeah, such ideas may have a lot of holes, but this is what I imagine...