Another thing that can happen is someone can do a conversion of some money from their traditional IRA to their Roth IRA. There is no dollar limit to conversions but they do result in ordinary taxable income.
So one way would be for someone to contribute $10,000 to their 401(k) at work for 20 years, retire, roll their 401(k) to a traditional IRA, then convert some of that to a Roth IRA.
There are probably other ways with other types of retirement plans and rollovers.
Another less common way is to contribute $5K to a Roth IRA 10 years ago, buy a stock that has done well over those years and then sell it to buy VTSAX.