Abigail is trying to put the shame of her sin behind her. Her sin was drinking a charm to kill Goody Proctor before the opening of the play. She and some other girls danced, there was nakedness, and a whole host of devilish activity. She has lied to her uncle about what went on. So, as with most young children, to cover up a lie, you tell a bigger one.
In this case, as Abigail got all the girls on her side, when it came time to place blame, she blamed Tituba. The adults got Tituba to name names of those who could be "witching" her, and then Abigail chimed in with great heart which caused the other girls to conform.
Abigail manipulated every piece of what happened. She lied. She blamed Tituba. She led the girls in accusing townspeople. All of this was done so that she wouldn't get whipped for dancing and conjuring spirits out in the woods.