This phenomena is discussed in a chapter of Freakonomics.
As women age, very popular names become associated with “old ladies” (such as Helen, Lily, Rose, Dorothy, Florence, Frances, Alice, Evelyn, Ada, Mildred, Ethel, Edna, Grace, Esther, Mabel, Gertrude, etc., all of which were some of the most popular names at the turn of the last century).
Because the names are associated with “old ladies,” parents stop naming their children these names, so they decline in popularity.
Eventually, the generation of women with these names age and pass away, and after some more time passes, new parents no longer associate these names with “old ladies,” because they never knew anybody with these names. So they start naming their children these names, which increases their popularity again.
This has already happened with names like Emma, Emily, Hannah, and Abigail. Parents of a generation or two ago would never have named their children these names. Today they make up some of the most popular names for girls.
The phenomenon is present, but less pronounced with boys’ names. For instance, John, William, and James have been continuously popular names for the last century. Others, such as George, Henry, and Clarence, have declined in popularity.