Casey has a boyfriend that is long distance in her senior year of college so they text and call a lot but the problem is that his name start with a D. So that's why she accidentally sends Derek a risqué pic. She was drunk on a Friday night and she was out with friends she took a selfie in the bathroom of the bar. She sent that and a text "I miss you" He being Derek never lets her live it down. See Derek has for most of his time knowing her had wanted to be with her had feelings for her but was managing it by ignoring it. So this picture had made it harder to for him to do so. So he would jokingly flirt with her only to say it was for someone else. Then one day he doesn't take it back.