The custom of carrying a bride over the threshold comes from the kidnapping practices of the Germanic Goths around 200 AD.
Generally, these men only married women from within their own communities.
But when the supply ran short, they would raid neighboring villages and seize young girls to carry home as their wives.
From this practice of abduction sprang the now symbolic act of carrying the bride over the threshold.
The tradition of the broom carrying the bride over the threshold originates from the traditional Roman practice of pouring libations of oils and fats, symbols of prosperity, on the threshold (doorway) of the couple’s new home. The oils made the doorway slippery, ergo the “carrying the bride across the threshold” tradition.