That's no bug, that's the difference between attacking a base or defending one.
Attackers start at A always, while defenders get to pick the base they want to start.
What you saw, we call a quicky. All defenders at B to move to A next round. Splatting the other guild at their base.
My suggestion is, to stay at A first round, and send a scout out (low power player) next round to B. If B is clear, get all to B. That way you can slowly look and take the bases in between. Slowly move up and keep communicating in guild about who is killed, how many left etc.
When you get to the point of owning a city, keep in mind that you can start wherever you like.
Good luck with guild warfare