I see this fairly often, too. It's not necessarily about attacking the country with only 30% of its bases left, though. That same country is often substantially outnumbered and the hordes usually attack against the least amount of resistance they can find. It's all about avoiding the fight.
This is indeed often true, but I might add that there are sometimes other factors also contributing to this, particulary map layout.
When it's easier to get into a fight/battle at one front than at the other due to terrain, distances between the bases, spawn points etc, players will go for the place they can get into action quicker.
A good example for this is the Trinity map.