Rainbow knew why Applejack wouldn't leave her boyfriend, but she didn't understand it. He was mean to her and treated her poorly. So on the late nights Applejack came over upset, Rainbow always made sure to treat her like she deserved.
On this particular evening however, more might be revealed that both girls were secretly hoping for to finally bring an end to this self-destructive cycle.

(The tag says sad but there is a nice ending, I swear)