Primary Driver for Suicides Among Middle-Aged Americans?

According to the CDC, the number of suicides among middle-aged Americans has risen by nearly 30%. From 1999–2010, the suicide rate for ages 35–64 increased from 13.7 to 17.6 deaths per 100K people. For men alone, the rate was 27.3 per 100K.

In your opinion, which is a primary driver for the rise in suicides among middle-aged Americans?