what story does nick recall about jordan, and what is the catalyst for his remembering in the great gatsby?

This happens at Gatsby's party. Nick recalls how Jordan cheated to win at one of her posh golf tournaments. Nick remembers this because Tom and Daisy had encouraged him to peruse a relationship with Jordan. Nick, however, had a relationship back home and wanted to break it off first rather than be dishonest.

This of course separates Nick from the vapid personalities of East Egg.