When Cassidy lets Cody know about her involvement in encouraging Adena about her crush on her tennis coach, Cody was mad that Cassidy encouraged Adena for Adena is sensitive and easily influenced by her emotions.
Cody may have been thinking about Adena's rash judgment from the first incident in 1x8, Mixed Up Doubles when Adena thought that Sebastian was interested in her, when in fact, he was interested in Megan.
In this episode we find out that Nate is at his mom's place, which explains why he didn't appear in the last episode or this one.
Slow burn may also be in reference to either passion or a downfall of an individual.
Usually it is used as a reference that when one does an action out of deceit, his downfall is to come slowly, with the consequences to be harsh, once the truth is out. But from time the deceit has been performed to the time they are caught, the person usually suffers from their own deceit - which is usually called guilt during the duration of their deceit.
Slow Burn is a title of a tv movie from 1986, about the main character in the mist of an intricate web of deceit and betrayal that leads to his downfall.
Squib and Adena go through trials in this episode that deals with deceit and betrayals that lead to their downfall. It is a parallel between the tv movie and this episode.