Continuity mistake: In the final scene, the position of the silver dollar lying next to Franklin's hand changes several times between shots. (00:23:45)
Continuity mistake: After Franklin falls through the window at the end, he lands on the pavement below - on top of the broken window frame. Next shot, when the police are examining the body, the window frame has moved out from under Franklin and is lying about a foot away. One shot later, though, it's back in its original position underneath him. (00:20:00)
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.