The absolute number one is reason for families to break up is resentment. Resenting the spouse you married, resenting how the children are raised, or resenting the financial situation of the family could be triggers to cause disharmony. For example, if there is a constant feeling of stress and irritation between the parents, small issues in life seem magnified. Most often than not, it is the children who bear the brunt of this discord.
Whenever life becomes difficult or when setbacks happen, couples start looking to change the things on the outside first. This comes to the second top reason why families break up - cheating. Trying to find satisfaction and affection from outside the marriage is a path to destruction of the family unit. For most partners, this kind of betrayal is too hard to deal with and they feel that separation or a break up is the only way to deal with this problem.
This brings us to the third reason for why families break up - breakdown in communication. When there is an inner turmoil of resentment and hard feelings towards your spouse, constructive communication is probably the last thing on your mind. The only communication that often occurs is yelling, fighting, or saying nothing and refusing to talk at all. Absolutely nothing can be mended if one or both partners refuse to discuss problems reasonably. Often a third party is the only option to open up communication.