My first thought was damage reflection (at least one person on Reddit said the same thing), but I seem to only die horrendously when I proc something, instead of on the first hit I land on the guy.
Same thing happened to my female Mechanic (although I don't have a video of that), I would just run around letting G-3 Raptor do it's thing, then the Patris's Lightning proc on the lv80 boss unique auto gun happens, and I just drop dead.