I figured it out :) (Destiny)

by CruelLEGACEY , Toronto, Friday, February 12, 2016, 19:43 (3301 days ago) @ CyberKN

It seemed to happen every time I killed the thrall as they were crawling out of their pond at the very back of the room. Firefly would still trigger on them as it normally would, but I would explode as well. As soon as they left that little cave at the very back of the room, I stopped exploding lol

*edit* I think it was happening when I killed any thrall as they crawled out of their pit... not just the explody thrall. But I'm not positive. More science is needed!