Continuity mistake: The flying saucer the old woman hacks to pieces with an axe is noticeably not the same one that lands in her attic in the beginning. A cruder version was built for the axing scene because the model used was a studio keepsake - the original spaceship from the classic sf film "Forbidden Planet." (00:23:00)
Continuity mistake: The laser burns on the back of the old woman's left hand disappear when the knife is slicing at her through the door. They return a few shots later, though. (00:10:30 - 00:17:30)
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.