An Occurrence at Owl Creek Bridge - S5-E22
Continuity mistake: After crawling from the water onto the shore, the Southerner lies on his back with his shirt gaping open, exposing his entire chest. But in the next shot, his shirt has fastened itself, completely covering his chest again. (00:16:00)
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.