The moor had a reputation for strange and sinister events. Angus MacRanald related some of these to Sarah Jane Smith to convince her of the danger of going onto the moor. He was certain that evil spirits inhabited the moor and had caused people to go missing or lose their minds. Sarah was unconvinced by his warnings.
The disappearances were almost certainly caused by the Zygons and their Skarasen to protect the secret of their existence. A UNIT soldier was crushed by the Skarasen while he was on sentry duty on the moor.
The Fourth Doctor was chased across the moor by the Skarasen, which was following its homing device, until Harry detached it from the Time Lord, allowing him to go unscathed. (TV: Terror of the Zygons)