The Putnams were a very wealthy, well-to-do family in the village who constantly felt like they were persecuted and judged. They were wealthy but constantly greedy for more land, and, recently, their selection for minister of the town had been rejected. To top it off, they had had many land boundary disputes with the Nurses, who had many, many children. Because the Putnams had many children die, they were resentful towards the Nurses for their many children, land, and respected status in the town.
It is on this backdrop that The Crucible is written; when Ruth, who was out dancing, is caught, she gets scared of getting in trouble, so starts acting strange. In Act One, where you can find all of this information, they describe how Ruth
"never waked this morning, but her eyes open and she walks, and hears naught, sees naught, and connot eat."
They immediately assume she's been bewitched. They say, "Her soul is taken, surely." They do not draw logical conclusions, like Ruth might be afraid, or acting, or being strange to distract them from her dancing. Even Rebecca Nurse recognizes that Ruth will get better "when she tires of it." No, Ruth must bewitched--this conclusion comes from Mrs. Putnam herself, who was the one who sent Tituba to perform a form of witchcraft to conjure the souls of her dead babies. It is a bit hypocritical for her to be accusing others of witchcraft when she herself was the one to rely upon it to find out who killed her babies.
I hope that those thoughts helped; good luck!