From a book called The Norman People (and quoted from the www.roperld.com website:

Roper, or de Rupierre. This family has been supposed to be descended from a member of the house of Musard, who is said to have assumed the name of 'Rospear or De Rubruspatha;' but there is no evidence for the statement. The name is derived from Rupierre near Caen, Normandy, the lords of which were of great importance in the 11th & 12th centuries (Des Bois). William de Rupierre (who came to England with the Conqueror) is mentioned by Ordericus Vitalis; in 1090 he commanded the forces of Duke Robert.

Yet another theory is that the name Roper was simply given to those who made ropes. So, pick an explanation (or make up your own) and it is probably as good as any.