That dragon (and every other dragon in GW2 I know of) came of his own will, because they are connected to their corrupted and saw that we were cleansing them. That makes them hungry. They need their corrupted (whether undead or whatever) to spread their corruption, and so they intervene whenever we do a good job of killing those off.
That is really the only think needed to summon dragons and champions. Or we can find them in their own lair.