Why can some alliance go directly to alliance center 2?

Alliances can take plots to advance towards an enemy center from a rally point or from any other plots that they have. The first rally point can be placed in the outer band. If your center 2 is near the outer band of the map then it may be easy for them to bridge to it from a rally point.

Or if they still have plots in your state from an earlier fight then they may be bridge to your AC2 from those. They may have had plots nearby and have been building towards your alliance center in the previous days to give them selves an advantage.