No doubt years in the making by now.
Raising kids can go a long way towards forcing you to stay together and make it work on the outside and at least you have a joint effort going in raising them. Now the kids grown and gone, and you look at each other and realize without the kids in the house there's really nothing left of the two of you. You somehow became only "mom and dad" instead of husband and wife.
Not as uncommon as one would think. I've known 2 couples personally who have divorced after 25-30 years.