Jack and the rest of the boys have been separated from their parents and are living "motherless" on an island. The incident with Jack's hunters illustrates the boys' frustration with being on the island alone, without any parental guidance or control. A mother nursing her babies is possibly the most "natural" act in all of life on earth. The separation of the boys from their parents is as unnatural as their action of killing the sow and putting her head on the stake.
You've selected a very good example of "author's choice" here. Golding could have had the hunters kill a pig, butcher it and put its head on a stake. Instead, he chose a sow (mother) and had her nursing her young when she was ripped away from them and killed. In the same way, the boys were ripped away from their parents. The fate of the nursing sow symbolizes the boys' situation on the island.
This episode occurs in Chapter 8 "Gift for the Darkness," and it is related in such a way that we react with horror and disgust. Sexual, or more accurately rape-like, diction is used in this scene to show the devolvement of the boys into savagery. We see here no noble man versus beast conflict; instead, we have a nursing, bleeding sow portrayed as the victim of the boys' frenzied bloodlust:
Here, struck down by the heat, the sow fell and the hunters hurled themselves at her. This dreadful eruption from an unknown world made her frantic; she squealed and bucked and the air was full of sweat and noise and blood and terror.
This episode is one of a series of parallel scenes in which a killing of an animal is involved. The first killing of a pig is barely mentioned as Jack and his tribe kill this one just as a ship passes by the island. The second scene involves the killing of a boar. This is the third one, and the most savage. The next hunt, of course, is a manhunt with Ralph as the prey.
The "trophy" of this hunt is the pig's head that is placed on a stick. It left to appease the beastie, in another savage ritual. This pig's head becomes a major symbol of evil and temptation in the novel, as it is this pig's head that talks to Simon urging him to "play" like the others.