As Private Wealth points out in "The Downside Of Sudden Wealth," the syndrome is not just due to quickly becoming wealthy. The syndrome refers to the common problem of people who become rich suddenly ending up with less wealth than they previously had.
There are several reasons for this phenomenon. The biggest, however, is that people who become suddenly wealthy do not always have the knowledge to handle their finances in a way that ensures there is always enough money coming in to cover obligations. For example, they might overextend on credit card or mortgage debt and not have the resources to service the debts when they have spent all of the money they received.
Knowing about sudden wealth syndrome is important if you are planning to leave a large inheritance to someone who has never before been wealthy. An estate planning attorney can help you to structure the inheritance in a way that makes the problems with sudden wealth less likely to occur.
Reference: Private Wealth (June 16, 2016) "The Downside Of Sudden Wealth"