Jess-Belle - S4-E7
Trivia: Jess-Belle was supposed to turn into a tiger instead of a leopard, but director Buzz Kulik auditioned dozens of tigers and told writer Earl Hamner, "Not one of 'em can act; can we make it another animal?" The cat that got the job had problems too. After Kulik put his crew inside a cage to protect them from the "vicious" beast, the mildly-tranquilized trained leopard missed his walking and leaping cues and kept falling asleep. (00:33:30)
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.