Continuity mistake: When Phyllis stumbles back toward the window, both its draperies are neatly in place. Next shot, just before she crashes through the glass, the drapes are suddenly half pulled down and a broken curtain rod that wasn't there seconds before hangs down over the window. (00:23:15)
Continuity mistake: When Joe goes to punch the TV screen, it shows moving lines and static. But in the shot where his fist goes through the screen, it's easy to see that the screen has been replaced with one on which wavy lines have simply been painted, and then backlit. The lines are still visible on the shards of glass after the screen is broken.
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.