It definitely seems to be related to using fire abilities. I've been playing my knight at max settings for a few days and it hasn't happened once. But if I use my Vanguard with Incendiary round it's constant.

My testing has shown some interesting results. It's never happens on my Assassin, no matter the location or settings. On my Vanguard it happens frequently, and more often the higher my Refresh/Frame rate. If I use maximum settings on my vanguard (240/200) it'll usually happen within a minute of using incendiary round or explosive surge. It only happens to me underground, or in an instanced area.

So I've done some more testing and it happens much, much faster after restarting when my refresh and frame rate are set at maximum. At 240/200 it happens almost instantly when I use incendiary round or explosive surge.

Same thing happening on my Vanguard. I played my assassin for a while the other day and didn't have this issue.