The Dutch word "van" is equivalent to the English word "of" and the German word "von". Hence it is not usually capitalized in names.
Buren is a city/estate in the Netherlands which was ruled by the Egmond family; Dutch nobles were commonly named after their possessions.
Literally translated, the Dutch word "Buren" means "neighbours". However the etymology following the name of the city ruled by the Egmond family is more logical.
Use of "van Buren" by the Dutch Royal Family
The name (van) "Buren" appears in the history of the Dutch royal house, being the family name of Anna van Egmond en Buren, the first wife of William I of Orange, the founder of the Dutch Royal dynasty. This use of the name predates the founding of the American Colonies.