Continuity mistake: The events shown are supposed to take place at almost exactly midnight, yet when Conny goes to the graveyard the sky is bright and everything is well lit, much brighter than moonlight. Just as he enters the graveyard there is an abrupt change in lighting and the scene is suddenly much darker.
Revealing mistake: As Connie approaches the graveyard the sky in the background is obviously a painted backdrop, as there is a visible seam.
Answer: It's explained in the episode. The aliens are experimenting to see how humans react to various scenarios. This just happens to involve them using super strength.
Thanks. It's been a long time since I saw the episode so I forgot.