The Lonely - S1-E7
Continuity mistake: Allenby brings Corey a stack of paperback books. He's holding them with the back covers facing out until the shot cuts, and then the stack of books has reversed itself in his hand: the back covers are now facing inward. (00:20:30)
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.