Unfortunately, as of now, there's not a definitive answer as to what exactly happened between the three former friends, but the trailer and context clues make it pretty clear that it has something to do with Kristen's relationship with her ex-boyfriend Brian Carter. Last season, Katie and Stassi really didn't like how Kristen was handling things with Carter and how she was treating them as a result of her tumultuous relationship. Namely, they though Kristen wasn't being honest with them about her life, which was frustrating and made it hard to connect with her. See: The song "Boy Problems" by Carly Rae Jepsen for further explanation. This lead to a huge blow-up during the season 7 reunion and, based on teasers, it seems that this same argument will escalate in season 8.