At one point, Jessica and some of her friends got Ella drunk. She then woke up in front of the entire Constitutional Law class naked. It was later revealed that Jessica did this because the classroom also happened to be the room of the professor conducting job interviews for a position they were both candidates for. Ella has evidently never moved on from this incident, and has held a grudge against Jessica ever since.
In "All In", Ella is the judge assigned to Harvey Specter's fraud case. She denies the motion to seal, knowing full well that he works for Jessica. She recuses herself from the case when Jessica finally admits to her past mistake.