If we assume this was not intentional, then the two most probable causes are human or technical or any combination of both.
A drone can't land, open Skype and allow an interrogator to speak with the locals in their language- it flies high, have limited visibility and the interpretation of the incoming data is based on prior intelligence.
If you look at the pictures you'll see that pine nut farmers looks the same as IS Jihadi terrorists, both use civilian clothes and sometimes they are actually the same person.