Euphorion's account of Laocoön is known to us only through a commentary by by Servius on the Aeneid 2.201:
ut Euphorion dicit, post adventum Graecorum sacerdos Neptuni lapidibus occisus est, quia non sacrificiis eorum vetavit adventum. postea abscedentibus Graecis cum vellent sacrificare Neptuno, Laocoon Thymbraei Apollinis sacerdos sorte ductus est, ut solet fieri cum deest sacerdos certus. hic piaculum commiserat ante simulacrum numinis cum Antiopa sua uxore coeundo, et ob hoc immissis draconibus cum suis filiis interemptus est.[note]
According to Euphorion, the priest of Neptune was stoned to death because he had not offered sacrifices to the sea-god to prevent the invasion. When the Greeks seemed to be departing, they wished to sacrifice to Neptune, they chose by lot the priest of the Thymbraean Apollo, Laocoön, as usually happens when there is no priest. He had committed a crime by having sexual intercourse in front of the statue of the god [Apollo] with Antiopa, his wife, and because of that his sons were killed by serpents. (My translation)
Euphorion's account of Laocoön's sin supplies the motivation for the versions of the story found in Arctinus, Bacchylides, Sophocles and the Apulian vase fragment. It could be that these versions all derived from an original in which Laocoön's desecration of the temple and subsequent punishment are paralleled with the corruption of Troy by its inhabitants and the consequent fate of the city.