Friends share secrets, that’s what keeps them close and liars are one tight knit group. I think it’s because they all share the same secret. They either witnessed or did something horrible (I know they helped in the blinding of Jenna Marshall, but I think there is something else like that but worse) They swore to each other and to themselves that they would never talk about it. They talked themselves into believing that nothing happened and they have done it for so long that they have surpressed the truth.
I think Alison’s answer to who she was afraid was a little cryptic. She basically told Hanna that the four of them combined remember more about that night than they think they do and that they are all better off with a really good lie. I think it was not WHO she was afraid of but WHAT….and she was afraid of the truth. she needed the girls to stick together and keep their secret in order to make it safe to come home. She even reminded them to shhhh.
I think that is why they are being targeted. -A wants the truth to come out and is determined to torture it out of them until they crack.