Discovered this by accident. Apparently at 4.0.1 halion is currently bugged - at least on 25m heroic. If you get the fire bomb / shadow bomb on you, you are 85% likely to get all (or most) of the next ones.
Just to give you example: first pull, i get 4 combustions
Second pull - another person gets 3 combustions, 1 on other person
Third pull - another person gets 4 combustions
Fourth pull - i get 2, another person gets 2.
Yeah, could be luck, but i doubt it.
Essentially tell the person who gets the first debuff to be ready to run instantly.
Edit: Forgot to add this was working in the shadow realm too
Edit: made the "4.0.1" part bold and underlined for some people.