There are a wide range of explanations given for why some Jews include an orange on the sader plate. The variety of narratives reflect how this practice, which has become popular in the last few decades, evolved over time to yield multiple tales and interpretations. 

Rebecca Alpert tells of a 1979 session on women and Jewish law presented to the Jewish Women's Group at the University of California Berkley Hillel by the  rebbetzin  of the campus Chabad house. One student asked the rebbetzin  her opinion about the place of lesbians in Judaism.  The  rebbetzin  suggested that lesbianism was a small transgression, like eating bread during Passover. Something one shouldn't do, but for which there were few consequences. Some time later, when the Berkley students were planning their sader, they chose to place a crust of bread on their seder plate in solidarity with lesbians who were trying to find a place in Jewish life. 

Others picked up this story, but struggled with the transgressive symbolism of bread on a sader plate. Professor Susannah Heschel was responsible for substituting a tangerine as a symbol for gay and lesbian solidarity. She then went on to share the story, and as it spread, it changed. The symbol became an orange, not a tangerine, and the focus on Jewish lesbians shifted to the place of women leaders and rabbis in Judaism. And this is the version that first began to appear in mainstream  haggadot. Today, let our orange be a symbol of inclusivity to all genders and sexualities. 

haggadah Section: -- Cup #2 & Dayenu
Source: Jewish Women's Archive