Parris is afraid that if people like Rebecca Nurse get hung, in conjunction with rumors of Andover's rebellion and Abby's bailing, that people will turn against him and the courts, and declare them frauds. So, he proposes that they should "postpone these hangin's for a time." Danforth refuses, but compromises by saying if Elizabeth can get John to confess, then maybe the people will be struck by a man of such "weight" actually confessing, and maybe John's confession will prompt the others to confess too.
Parris thinks that Abby and Mercy left because "they fear to keep in Salem any more", which means that they fear that people will turn against them. Since the rumors that Andover had overthrown the courts and turned against the accusers, Abby was fearful that would happen to her, so she bolts, stealing Parris' money first.
Reverend Parris is very concerned for his safety, especially in Salem, as he's had many signs that people are not happy with him or the handling of the trials. First, very few people attended John Proctor's recent excommunication; Parris fears that it "speak[s] a discontent." Then, there's news of the rebellion in Andover, and this news is affecting Salemites. Now, his niece, Abigail, and her friend, Mercy Lewis, have robbed him of his life's savings and fled the town, and finally, when he opened his door to leave his house, "a dagger clattered to the ground." He is now afraid to leave his house at night. He fears riot, especially if someone the likes of Rebecca Nurse or John Proctor hangs because they are so well respected in the village. He asks for a postponement, and he is hopeful that Mr. Hale will be able to convince some of those condemned to hang today to confess, if given more time, and this will cast doubt on the innocence of the others. Parris feels that if they all go to the gibbet claiming innocence, his and Danforth's "good purpose" will be "lost in their tears."
Abigail is a smart girl, and she sees the same "discontent" her uncle sees in the village, I'm sure. If she feels the tide of public opinion to be turning against her and in favor of the convicted, she's intelligent enough to get out of Salem while she still can. She is likely motivated by fear -- fear of being known for the liar she is and fear of being punished for it. She is also motivated by self-preservation; she's not going down with this ship, so to speak. Salem has, apparently, become a wasteland. As Hale says, there are crops rotting while orphaned children and neglected animals wander the streets. If she saw an opportunity to escape, why wouldn't she take it now and save herself?