Gameplay. 360 degree rumble flame spitter?

I've been watching a few games lately on the diamond+ circuit, and I see rumble fairly often, and I noticed something kind of odd. When rumble used his flame thrower, and then uses zhonias, if the flame thrower is still going, the damage seems to become 360 degrees around, instead of just one direction. I noticed it because I was watching him fight a riven, and when he used hourglass, his fire was going, riven was behind him, and riven went from around 300hp to dead. I was watching carefully. There was no clear explanation. Lyandris wouldnt do that much, and there was no ignite, and rumble had used his ulti previously in a different location, and it was just those 2 champs around. I noticed that after I saw that in that game, the guy was using his zhonias EVERY time with his flame spitter, and get several kills with it. It's the first time I saw it was this game, none of the others before it had it(not that i noticed atleast, I dont usually watch rumble, im usually focused on other champions to pick up tips and tricks) Wondering if maybe I'm just seeing things(but then why did it happen several times?)