Discovered this with a buddy last night. Volatile armor tooltip says it returns X damage to melee attackers. My buddy and I dueled and he walked to max bow range. I had volatile armor up. When he heavy attacked me, it returned returned damage to him. In fact, the game counted it as DoT damage because it triggered skoria meteors. I was doing nothing else but standing there taking his heavy bow attacks (we were testing different penetration values and how they impact damage output).
Interestingly, we also tested resto staves and inferno staves, and neither one triggered the damage return. Only bows.