Local officials said the dead included two children.
The mayor of the township of Jocotan, near the Copan ruins in neighbouring Honduras, confirmed the deaths. The area, located east of Guatemala City, is poor and people frequently ride in the back of pickup trucks.
Mayor Juan Ramón Díaz said the truck suffered a breakdown on Wednesday and the driver got out to fix it.
But he may have forgotten to set the emergency brake, because the truck rolled backwards and plunged into a small canyon.
The 20-year-old driver was arrested amid fears local residents might attack him. Vigilante attacks and mob justice are common in Guatemala.
In a separate incident, four men and two women were shot to death on Tuesday at a water park in the township of Tiquisate, south of the capital, officials said.
Local media said the victims had been suspected of stealing a truck, though officials did not confirm the reports that it may have been a retaliatory attack.