Many of these people may have separated before the divorce became official, which would help explain why wealth starts falling so early. Divorce is often a long and messy process, and you can see this in the four-year decline in wealth.
Perception does not match reality. People don't want to admit they're doing terrible.
Some of that effect is due to economies of scale ... that is, two people can live more cheaply than one.
We can't tell from these data the reasons why divorced people have so much less wealth than those who are married. If you really want to increase your wealth, get married and stay married.
Some people may also be working less and not trying as hard to build wealth as they have marriage troubles. Divorce is often a long and messy process.