From the little information we got, what happened to Lindsay sounds very traumatic and I can understand why she's having a tough time handling everything. She's carrying a lot of guilt with her for being the only one to survive the attack, but maybe it's something else too. Maybe she feels responsible for her friends' deaths. I'm am also wondering why it is surfacing now. I think that she believed she had moved past it, and in "Manhattan Manhunt", she was proving to herself that she could handle it. I think it's been bothering her now because something from that event has come up and she's going back to Montana to figure something out, pull herself together or even help with the investigation. That, or something from one of her previous cases triggered a horrible memory that is bringing everything back (possibly the mermaid episode or the close call she had in "Not What It Looks Like"). There is so much we don't know about what happened and hopefully we will find out very soon, such as: Who killed her friends? Did she know them or was it a random act of violence? If she did know them, were her friends killed to get back at her? Looking back at the episode where she went to the jail, obsessed to know why the killer murdered Sarah, makes me think that she was a victim of a senseless massacre. Where were they when this happened (studying at the library, hanging out a friend's house, partying, or loitering behind an alley)? How were they killed and was it immediate or torture (even rape)? Why did she live? Did she barely survive or was she spared? Lastly, are the killer/s behind bars, dead, or still roaming free? I think those who said that she has issues with mothers because the mothers of her friends blame her for the incident and being the only one to survive may be right on.
Just some theories I have...we better find out soon. They've dragged this on too long.