Strong family history and genetics are well known to be some of the causes of varicose veins.
Trauma is another cause that often gets missed. Especially if it is only in 1 leg.
This trauma could be in sport, repeated knocking of the legs, surgery in and around the region or even a previous motor vehicle accident or work injury.
They may not be visible immediately and sometimes are only seen many years later.
They are often seen in the region of where the trauma occurred or further along the leg.