What is the relation between the Human Development Index (HDI) and Number of Suicide cases in the USA?

The link between HDI and suicides is not well understood. However, let us examine the impact of rising HDI on suicide rates in the United States.

The graph for the United States indicates that as HDI climbed, suicides increased until a certain point, after which there was a sharp reduction. Following then, it began to rise again with HDI.