Although Hulk's temper stayed largely in check around the Defenders, his rampages continued when left on his own. It was only a matter of time before new heroes popped up to drive off the Hulk. Enter: Wolverine.
Their historic first encounter established that the Canadian hero was tough enough to keep fighting after Hulk tossed him around, and that Wolverine's claws were sharp enough to penetrate Hulk's gamma-tough skin (Incredible Hulk 180-181). There was a chance Wolverine might win.