Why should the game influence things in the favor of the guys with the enormous numerical advantage?
"The game" does not influence anything in favor of anyone, or against anyone. The game responds to the situation set up by the players. It doesn't care who is who, or anything like that. All it knows is that Realm X is at war with Realms A, B, C, D, E, F, G, H, and I. And that the peasants of Realm X happen to *like* Realms A, B, C, D, E, F, G, H, and I. So regardless of how the war happened, they are going to be pissed off at their government, either through "Why the hell did you declare war on all these guys, we love them?!" protests, or "How could you be so stupid as to piss off so many of these people that they all declared war on us?!" protests.
Certain people completely ignored some basic mechanics of the game, and ignored or misunderstood what the game was telling them. Proper preparation via diplomats/ambassadors or priests
(oh wait, that's right, religion is, for all intents and purposes, illegal there, regardless of assertions otherwise), having courtiers and other non-combat roles standing by
(oh wait, that's right, that realm doesn't believe in non-combat roles), lowering tax rates to something that doesn't cause the peasants to be pissed at you to begin with, and, how about this: maybe NOT planning a secession a day before you *know* the !@#$ is gonna hit the fan.
There is a huge confluence of negative factors all coming together at the same time that caused this to happen, and many experienced players could have predicted something like this, given a good idea of the sequence of events that were going to occur. You
cannot ignore all the facets of the game you don't feel like dealing with, and expect nothing to come of it.