Continuity mistake: No one drinks much lemonade, so there's neither time nor need for Eve to refill the pitcher during the brief cut to a scene on the porch. Yet after she has emptied its contents into the glasses on the tray, the lemonade pitcher refills itself. (00:15:15 - 00:19:10)
Visible crew/equipment: Right after the phone conversation, everybody starts going towards the door. As they do, if you look at the couch on left you can very briefly see a shadow of something moving across it. It couldn't be any of the actors since they were all either outside or on the other side of couch.
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.