While it would appear that retirees are returning to work because they have no choice financially, it actually turns out that most people return to work voluntarily, according to The New York Times in "Many Americans Try Retirement, Then Change Their Minds."
It also is thought that retired people stay retired. However, over the last couple of decades, studies have found that an increasing number of retired people decided to go back to work. One study concluded that 40% of working Americans over the age of 65 had previously retired.
This trend might be the result of people living longer, healthier lives and work being less physically demanding. Another reason is that the new job is more flexible than the former job. That also suggests those who return to work are generally better off and do have the ability to make the choice.
People who have more money and who work in less physically-demanding jobs live longer and healthier lives than others. However, some may not return to work because they have health issues.
Reference: New York Times (March 30, 2018) "Many Americans Try Retirement, Then Change Their Minds."