Marriages don't usually sour overnight. Instead, they tend to slowly fall apart, with each spouse gradually becoming more independent and thinking of himself or herself as an individual, rather than part of a couple. That's often reflected in their finances as well, because it isn't uncommon for one or both spouses to have hidden a little money away for a rainy day without telling the other person.
California's laws, however, make it very clear: In the event of a divorce, each spouse has a duty to disclose all of their financial information and assets. There's several different reasons for this: