My theory is that Cece was responsible for the explosion of the house of Jenna. Cece knew aboutv Ali's Offer to Noel: He should get the photos o Ali and Cyrus from Jenna's house. Maybe Ali told her - or Cece observed both - When Alison told him about the photos.
After the explosion - Noel does not trust Ali anymore. He thinks Ali wants to get rid of him.
And the 4 girls trust Alison not a bit anymore.