The English surname Sowden is of local origin. Local surnames were derived from the place where a person once lived, or a prominent local geographical feature, perhaps a hill identified one person who lived near a bridge, river, Lake Etc. Many of these names were preceded by a preposition such as “de”, “at”, “by” or “in”.
These tended to lapse in later years. In this instance, the name denotes “of Suddon”, ie. the south down. This is a well known West-Country name.