I am not an expert in patronymics, but I can give you some sensible thought on the subject.Here it goes.
Before there were surnames, most communities used only first names.To avoid the confusion of hundreds of Johns, Abrahams and etc, these communities began using prefixes and suffixes to differentiate between people with the same given name. The Hebrews used the prefix Ben to mean son of.Sometimes the Arabic Ibn was used.The compound names became the surname.For example Joshua Ben Abraham, or William John Son.
In the former case the prefiix was dropped to read Joshua Abraham or Josuha Abramson.
I must assume that an ancient patriarch of the Absalom family was named Absalom and it became the family name.
The only place I have ever seen this name as a surname is in the Catholic census records of the Jewish community of Spain.