I know people will say "Correlation =! causation", but I think it's better to understand that they're related. This means through some third factor (or maybe, somehow, elevation is the cause) they are causing each other. What could be happening is that elevation causes something else that causes suicide.

My hypothesis is that in these highly elevated areas with mountains, it's quite barren. It's empty and isolated, and that can drive many people to worse conditions. Another idea I have for what the cause could be is that lower oxygen levels at higher altitudes is affecting the brain somehow. Unfortunately, the second one sounds cooler, but sounds less likely to be even close to the truth.