The dipping of greens is reminiscent of the historic "dipping" that led Israel into exile in Egypt. The descent to Egyptian slavery began when Joseph’s brothers sold him into slavery and dipped his coat of many colors into a slaughtered goat’s blood in order to mislead their father Israel about his beloved son’s true fate. The ascent from exile – moral and physical – began when every family gathered together with their neighbors to share a lamb on seder night and to dip in its blood a hyssop plant and to dab it on the doorposts and the lintel as a protection against the tenth plague. The "dipping" also therefore refers to the "dipping" of the Egyptians that was complicit in the story of liberation and return from exile. 

haggadah Section: Karpas
Source: A Different Night Family Participation Haggadah