There are many reasons why a man cheats on his wife, so there could be several possible reasons why your father cheated on your mother.
However, there is no justifiable reason for this as a husband is meant to be loyal to his wife regardless.
Some of the reason Men cheat on their wives are mentioned below;
1. Maybe he was forced to marry her
2. Maybe the wife is a nagging wife
3. Maybe the husband stopped loving his wife.
4. Maybe the husband is no longer interest in the marriage
5. It can be as a result of change in character or change in body size.
Regardless of this, a man should not cheat on his wife and if this happens the couples should forgive each other if they can and move on.