Many people have wondered how those combo attacks work in GW2, because something is apparently doing something in all those explosions during the event. Conveniently, the GW2 manual was posted and has a section on combo attacks. It says almost nothing and recommends experimenting. (If you are reading this in 2016, and that link gets a third sentence, hello people of the future!)
The wiki, as expected, actually explains things, and does so in a beautifully simple table with links to what skills create those combos. (If you are reading this in late 2012, and the wiki is reliably available after the crushing launch week traffic, hello people of the future!)
This explains all those “Area Healing” pop-ups during that “fight the champion giant” event.