Stopover in a Quiet Town - S5-E30
Continuity mistake: Bob's cigarette sets the artificial grass on fire, and he rapidly stamps it out. When he finishes extinguishing the flames, the grass is scorched, but intact. He doesn't touch it again, but one shot later, the burned grass and all its ashes have vanished and a huge hole has appeared, revealing the metal mesh underneath. (00:13:50)
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.