My idea is that only two Siegestars of the Juggernaut class had been built due to their size. The Juggernaut was the first, and was followed by the Dreadnaught. While the Dreadnaught was lost in battle with Cylon forces, the Juggernaut herself was propelled into an alternate reality when a nearby star went nova and created a rift between dimensions. She came out in the nBSG reality.