I want to point that beyond all reasons like "walking past of least resistance" (which indeed does happen, I'm not going to argue this) there are sometimes other things causing such 2-1 situations as well.
For example distribution of starting bases after log-in. Many players don't really scan the map thoroughly after logging in. They just head for the fight closest to their start base. If know Rooks & Bish have their respective starting base (The one with lowest number) on the Knight front, both will start to attack Knights in big numbers without any consideration or hesitation. You can observe this effect quite well on maps like Baltic, NDisles or Sfma (with it's eternal A1-A2 Furball)
Another one the dynamic of the war. Moving fronts sometimes result in one front having much closer distances between opposing bases than the other one, or with less obstructions like 20k mountain ranges.
All those reasons can add up, then throw in some obvious gregariousness (often called "hording"

) and the result is one side getting thrashed.
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And on the "that is what the arenas have become"... It's not a recent trend, it's exactly the same way it had been 4 years ago when I started.
In some respects, it's even less worse. Players seem to forget how things were working under the old "win teh war" conditions.