What brings Rip to deep sleep?

To escape his nagging wife, Rip often wanders off into the Catskill Mountains to hunt. On one such expedition, he comes across the ghosts of Henry Hudson's men. Hudson was an English explorer of the early seventeenth century, famous for his voyages around modern-day New York. After hearing the sound of thunder, Rip follows Hudson's men deep into the wilderness. While the men play a game of nine-pins, Rip drinks a particularly heady brew that sends him into the deepest of deep slumbers.
The mysterious purple liquor drunk by Rip clearly has magical properties, and it's these properties that send him off to the Land of Nod for the next twenty years. This is just one of the many folkloric elements in the story. Magic potions of some kind or another are commonly used in fantasies and fairy tales. They offer characters a means of escape from an often harsh, uncomprehending world. Rip Van Winkle feels completely at odds with the society in which he lives, and it is only through wandering off into the Catskills and deeply imbibing the magic sleeping draught that he can escape it completely.