Abby, who still has strong feeling for John Proctor, confesses that the events that took place in the forest involved simple dancing and charm rituals, or as she refers to it "sportin'". Proctor still has feelings for her as well, although he is much better at controlling them.
Dancing in those days was considered sinful, a sign of possession by the Devil or evil spirits, so being found out could lead to serious trouble, perhaps a public whipping. So the girls lied about it to protect themselves and the lie just grew and grew.
The involvement of Tituba, the slave, in the performance of charm rituals would have been even more serious, and for this reason, they blame all of the events initially on her.