It's like the skunk gives off two kinds of poisons. Once you explode the animal, it gives off a little bit of poison that is like a "second poison", it counts as a new smoke. If both poisons reachs a worm it will result as -10 hp.
Example: when you manage to skunk 2 worms you can see it's -5 on a worm and -10hp on another worm most of the time, the worm that gets -10hp is the one near of the skunk explosion
Hope I could help, wasn't easy for me to explain this shit in english xD