Keg Toss will trigger (Shoes up in combat log) and remove Stunning Repurcussions but did not stun the champion that used Keg Toss. Keg Toss unit did not have Unstoppable.
Stunning Repercussions triggers if an aoe attack is *clicked* on the champion or in the case of Bombs, centered. Subsume and aoes work in the same fashion. Even works with friendlies as seen here: (I cast Repercussions on Skeezick Vanguard and then centered the Acid Bomb on it) Is this WAI? Either way, this doesn't explain the Keg Toss champion avoiding the stun, don't have one to test with.