it isn’t stated explicitly. the odyssey tells us that circe stirred baneful drugs into their food, ἵνα πάγχυ λαθοίατο πατρίδος αἴης, so that they might completely forget their native land. the men’s bodies and voices become the bristle-haired bodies and rough grunts of pigs; αὐτὰρ νοῦς ἦν ἔμπεδος, their minds remained unchanged. previously, circe has turned men into wolves and lions; transforming odysseus’ men into lowly filthy pigs seems to us a particular insult—but we’re reading through modern eyes.
it’s believed that for centuries pigs were regarded as a sacred animal of a vegetation/fertility goddess with paleolithic origins. in northern greece many sow effigies and pig-shaped vases have been found, suggesting that they were integral to cult practice. terracotta figurines of pigs and women with headresses have been found on the slopes of parnassos above delphi and in the corcyrian grotto, predating worship of apollo. it seems likely that the rounded bodies of pigs were symbolic of seed/field fertility in near-eastern cultures at that time. in ancient greece pigs were considered demeter’s animal. they’re sacrificed to her because pigs’ blood is used for purification—there’s a vase-painting of apollo cleansing orestes of the pollution of matricide by raising over his head a bleeding pig—and they’re involved in rites celebrating her power (e.g. the thesmophoria, a festival of fertility performed solely by women). however, pigs are also tied to stories of male sacrifice for powerful goddesses, e.g. babylonian king tammuz, lover of ishtar, is killed by a wild boar; aphrodite’s favoured mortal, adonis, is gored to death by a boar; osiris, twin and lover of isis, is castrated by set disguised as a boar. these stories affirm female power: she will bring about his resurrection.
if circe as a symbolic figure contains vestiges of those earlier mythic entities, i think there’s an implicit notion of punishing/making manifest the sailors’ bestial natures (n.b. odysseus doesn’t resist circe’s magic because he’s noble and upright; it’s because he’s cunning and god-favoured); but also a recognition of circe as a danger to masculinity—circe “unmans” odysseus’ men, makes of them sacrifices to her feminine power; after odysseus rushes at her with his sword, she relents and agrees to free them, but the payment she demands from odysseus is sex, and odysseus fears that she will steal his manhood, requiring her to swear an oath that she will not cause him harm; when she restores the men to human form, they are younger and more beautiful. witches in the odyssey are strange, liminal, indeterminate, hybrid entities; it’s a story of men, and in the eyes of men they’re half-divine half-monster—sorcery and softness and threat.