People do the exact same thing in Sophia normal too, which I used to run a lot just because I felt like it. The mechanic IS pretty confusing to see just out of the blue they probably thing it's like a burst aoe or something that they need to be by themselves for. Your options there are to let them eat it, make a macro to explain it while its happening, explain it during the pull before the boss, or rescue someone into yours if you're healing.