So I was running the Vault today, we had Atheon's health about 3/4 of the way down in the third teleporting sequence, I screwed up my ground pound and the accursed health-regenerating Minotaur killed me. My teammates grabbed the Relic, but alas, it was too late. As we waited for the inevitable restart, this happened.

Exhibit A

Atheon apparently decided he didn't feel like living anymore.

I think he actually suffered from severe depression.