I think it's something similar to some silly other AI will do. A corvette pops in their space, and they will go stackwipe it with all their fleet. With many AI's doing that, it keeps them occupied for quite some time. An empire awakened to stop them too, and that one couldn't beat the Unbidden either. They certainly had the fleets to expand their blob further but didn't do so. I got my fleet wiped by them, took me ages to rebuild a bigger fleet, but Unbidden were still hanging around in their space by the time the new, better fleet arrived.
That was before the anchors, though. More recently, we only had the Unbidden spawn in our space from early Jump Drive so we always had to crush them at once!