Here is what I see happening: early on, the people in a republic are united in their efforts to overcome adversity (harsh environment, lack of food, low living standards). Since they are all working together, they eventually succeed in creating a better place for everyone to live in. Their descendants, however, have never seen true hardship and take the high living standards they have lived in for granted. Having nothing to work for in the way of improving their society, they turn against their society. They imagine that they can destroy the society that they hate while maintaining the luxuries that the society provides.
I'm sure that every society in the past that destroyed itself had its share of citizens who saw what was happening and tried to work against the destruction. But the ignorant masses ignored them.