Why did Abigail lie to her uncle about dancing in the woods?
As a member of a theocratic Puritan society and the niece of its reverend, Abigail realizes that her actions in the forest will result in severe punishment. She apparently fears being whipped, excommunicated, or, even worse, executed for consorting with the Devil. She is also acutely aware of her uncle's (Reverend Parris) uncertain position in the town because he consistently reminds her of the attempts of a faction to get rid of him. If she should be found wanting in matters of belief and adherence to the church's strictures, her uncle's enemies will pounce on him, leaving Abigail herself in trouble.
Abigail insists that their actions in the forest were fun and games. They were not consorting with the Devil and were participating in a meaningless activity that was led by Tituba, the Reverend's servant from Barbados. Abigail, who comes across as being quite mature for her age, is intelligent enough to realize that she and the other girls are in serious trouble. It is quite easy for her to make Tituba a scapegoat. When Reverend Hale asks her, close to the end of Act I, if she summoned the Devil, she is quick to blame Tituba. When Tituba, fearing for her life, starts naming others as witches, Abigail is equally quick to follow suit. She starts shouting out the names of those she supposedly saw with the Devil. Her accusations encourage the now awakened Betty (Reverend Parris's daughter) to do the same.
Soon all the girls involved in the shenanigans in the forest do what Tituba, Abigail, and Betty have done. The town is in an uproar about the accusations, and many innocents are arrested. The girls have discovered an efficient way of ridding themselves of guilt. They become the court's prime witnesses, and Judge Danforth sees them as instruments of God being used to rid the town of Satan's malevolence.
The above events expose two distinct ironies. First, Mrs. Putnam confesses that she sent her daughter, Ruth, to Tituba to make contact with her dead babies. She, however, is absolved of any guilt and, to add further irony, becomes one of the chief accusers in the witch trials. Second, Reverend Parris knows that his niece, Abigail, is not as untainted as she presents herself. He knows that the Proctors dismissed Abigail and that no one in the town is prepared to employ her, but he continues to support her. He becomes so immersed in the trials that he, just like the Putnams, becomes one of the court's most expressive and enthusiastic supporters. The Reverend, just like the girls, wants to save his skin.
The culture in Salem is very conservative as a result of strict religious doctrine. Abigail tells her uncle, Parris, that the girls were dancing in the woods because this will elicit far less punishment than the act of communing with the devil and/or the dead. Mary Warren warns them that witchery often leads to hanging, whereas dancing only leads to whipping. Abby threatens the girls to go along with the story that they were dancing and that it was Tituba who conjured Ruth's dead siblings (at Mrs. Putnam's, Ruth's mother, request).
Unfortunately, no one in an authorial position seems willing to place some blame on Mrs. Putnam. More significantly, Salem is so ensconced in strict religious beliefs and fear mongering that they slide quickly into hysteria. When accusations of consorting with the dead come up, Abby is quick to blame Tituba. And Tituba claims that the girls begged her to do this. Eventually, Abby blames others. Abby uses the dancing excuse and blames others in order to avoid being accused of witchcraft herself.