Head hunting in the Cordillera was not done at random by our Igorot ancestors, there were various reasons, a broken peace pact, a serious personal slight, a dispute involving rape, murder or theft caused by a visitor or a rival from another tribe. A vendetta is declared and revenge becomes a matter of tribal and individual honor.
Other reason include the desire for abundant harvests of cultivated products, the desire for exaltation in the minds of descendants, to increase wealth, to secure abundance of wild game and fish, to secure general health and favor at the hands of the women, and to promote fertility in women.
Igorot Holding His Trophy (1909)
Usually, the practice was rather like a ritual or test of manliness. For instance, in an ethnoliguistic group, men could only marry after they were able to cut off one's head. But they didn't cut any random person's head, rather they did it on a one on one duel. Being able to cut a head meant they've "graduated" from boyhood and they were strong enough to protect a wife and family.
In the Igorot head hunting practice, most heads of their enemies were cut off with a battle-axe even before the wounded man was dead. If one warrior at the scene had never taken a head, he would be allowed to cut this one from the body and thus be entitled to the head taker's distinct tattoo (Chaklag).
However, the individual possession of a freshly severed head had to be "activated" through proper ceremony and ritual before it would "release" it's spiritual and magical powers.
In order to try and stop the practice of head hunting, the Americans started to organize sport events here in the Cordillera. You see a lot of pictures of tug of wars, relay races and baseball games going on -- an attempt to stop the constant murders that were taking place throughout the region.
Igorots stopped the practice of headhunting and there's no concrete evidence that our ancestors practiced cannibalism.