Alliances are an essential part of the mating strategies of the males. First-order partnerships will single out a female, rush at her, and then herd her away to have sex, which is coercive (this is a general assumption, because it is rarely seen). During this aggressive corralling, the female repeatedly tries to escape, and does so in about one of every four attempts. The males restrict her attempts at freedom by charging in, and bashing her with their tails, head-butting, biting, and body-slamming her into submission.
Second-order alliances do the same, but the team-up makes a ratio of five or six males to one female. The males are often closely related in these alliances, so as a means of transferring their genes into the future, this fits perfectly well within evolutionary theory. On occasion, they form looser “super-alliances,” where multiple second-order gangs will join forces—up to 14 individual males—to corral a single female. These gangs don’t tend to be closely related.
It should be noted that forced copulation has not been directly witnessed, as far as I am aware. The evidence comes from observations of the pre-copulatory behavior, and physical evidence of violence on the females.