Here’s a study on income variability, indicating that less than half of top 1% earners (40-43%) are still in that group 10 years later https://www.treasury.gov/resource-center/tax-policy/Documents/Report-Income-Mobility-2008.pdf
One might surmise that (as @Happytimes2001 notes) quite a few people get into that group through one off windfalls in a particular year (large stock option/business sale gains for example) and that others get into the lower end of that group for a few years at the peak of their careers.