Strip,
I don't think it sounds like the Bish were hordeing and had the numbers to begin with. It's that one country was getting beat back, so as they got pushed into the corner a sizable number of their team decided to go to a different arena where the field numbers were more balanced (or they had the advantage) CREATING the horde.
I think this is one of the biggest drawbacks of the multi-arena format. If one side gets the upper hand, rather than the down country's members all logging and going to a different arena driving up ENY as a result, they would actually stick around.