Continuity mistake: Renard gets his bottom wet from sitting down and tying his laces on his new shoes. But after he is hit by the car, his trousers are dry.
Revealing mistake: In scene just after Rod Serling's opening narration, you can quite see that scene is being played in reverse because smoke is coming downwards back into the man's cigarette.
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.