The girls were dancing naked, drinking blood, and calling on spirits in the forest.
While to us these antics may seem harmless and rather silly, they added up to serious charges in Salem.
In Salem, this is witchcraft.
Any of these activities could have gotten them all into quite a bit of trouble. Put together, these actions will be seen as dangerous and dangerously taboo by the community. Abigail knows this and, like Parris, she knows the punishment for such actions is extreme.
Parris strongly suspects the girls are involved with witchcraft. He knows not only that witchcraft is punishable by death...
Abigail lies to cover for these taboo acts, smartly, diverting attention from the girls so that they might avoid being punished for witchcraft. Her motivvation for lying, then, is primarily to avoid the pain of punishment.
Abigail lies for the same reason a lot of children lie when they are doing something "naughty." She doesn't want to get in trouble. While dancing with a bunch of other girls in the forest might seem tame by today's standards, that was a very taboo activity in Abigail's time period. It was something that good Puritan girls and ladies just did not do.
The other reason to lie about her activities in the forest is to save her own life. People in the town, specifically Parris, suspect the Devil is the root cause of the odd forest activity. Abigail absolutely does not want her activities to be thought of as witchcraft, because she knows that she can be hanged for suspected witchcraft.
Parris, pressed, turns on her: And what shall I say to them? That my daughter and my niece I discovered dancing like heathen in the forest?
Abigail: Uncle, we did dance; let you tell them I confessed it - and I'll be whipped if I must be. But they're speakin' of witchcraft. Betty's not witched.
As you can see, Abigail is perfectly willing to be accused of dancing and being punished for it, but she absolutely doesn't want "witchcraft" associated with her.