Money comes and goes in waves. Almost everyone on earth has had their finances fluctuate in some way, you lose a job - you have less, you get another one, and you have more. But most people don't experience a massive class change in their lifetime, whether upwards or downwards.
There are international studies that show American class mobility is more myth than reality, and most people die in a similar class to the one they're born to financially. In fact, with rising inflation and the housing crisis, it's far easier to lose financial stability than gain it.
In a popular Ask Reddit thread, people who were wealthy then "lost it all" or knew someone in that boat share what happened.
1. From HorsePaard:
My driving instructor used to be rich. He had a gym inside a shopping centre in the centre of the city. Then he did a stupid deal where he tried to sell it and didn't get all the details in writing. He intended to be partial owner, but he messed up, so the new owner simply kicked him out.