The gossip
Skylar wrote that she felt very uncomfortable. She had been the third wheel for a while and now that the two had been kissing intimately, she felt even more left out. This was written from Skylar’s perspective, of course, so we don’t know how she reacted that night. Did the fight start there?
Could it be that she reacted so disapprovingly and threatened to tell Rachel’s Catholic parents? Or to other people in their environment? Making her disappear for that reason sounds very extreme, so the detectives initially assumed the overdose story.
Later it turned out that the friends knew more about what had happened to Skylar…