When someone decides that the time has come to change their career, it can be as a result of a number of things. For some people, a change in career may have to occur in order for them to better align their work life to their personal life.
A change in career can also come about as a result of someone deciding that their current work benefits are not good enough, which forces them out of their current sector and into another, where the benefits fit their expectations. You can also read more about changing careers in your twenties online.
Many people also experience the realization that they never really wanted to do their current role, they just fell into it and so when this hits them, they decide to pursue a career that they were actually always interested in, but never got the chance to do before.
This is also true of people who go back to studying, upon completing their course, they realize that they are now qualified to do an alternative role and so want to make the most of what they have learned.
Money can also be a motivating factor that causes people to change their careers, especially if the new career does not involve a huge amount of time spent retraining, although, for some, the possibility of retraining in addition to a larger salary has huge appeal.
On the flip side, many people, over the past few years, have discovered the value of having a stable career and a consistent stream of money coming in, so it may be that self-employed plumber decides to change career and get a 9-5 office job instead.