Typically, a "gray divorce" is one in which you and your spouse are both over 50. Really, though, it refers to any divorce that happens later in life.
One key event that could trigger such a divorce is when the kids to go off to college. When the last one leaves the house, when the parents watch him or her walk into that dorm room and a new life, they often decide to split up.
Why does this happen? Obviously, every relationship is different, so no answer is going to fit all cases.
However, one main reason is that some people have been feeling like the marriage is broken for years. A spouse is controlling and jealous. The spouse has a problem with alcohol addiction or gambling. The two don't connect like they used to. The spouse is emotionally abusive and distant, even if it never quite gets to the level of physical abuse.
When the kids are still living at home, though, people feel obligated to stay together. They're doing it because they know it will help the kids to have two parents in the house. It's not what they want, but they're making the decision based on the kids' needs.
As those children move out, though, they feel like the necessity of the marriage ends, in that sense. Now they can choose if they want to be married or not, based on nothing but their own desires. In a toxic or even abusive relationship, they may very well choose to end it.
Of course, splitting up late in life can be complicated, especially as retirement nears and asset division becomes so important. Make sure you know all of your legal rights.
Source: Huffington Post, "The 9 Totally Legit Reasons To Get A ‘Gray Divorce’," Shannon Bradley-Colleary, accessed Jan. 19, 2018