There are actually numerous fanfics where Issei and Rias are betrayer, with the entire ORC members often acting out of character and becoming betrayers. One type of betrayal plot I've never seen before in DxD fan fiction is one where the villains, like Diodora, get betrayed instead of the main characters.
But that said, Rias is often used as a betrayer in fanfiction because it aligns well with her character, and this is evident in her behavior and the words she uses when resurrecting Issei. Additionally, her appearance in the Vatican during Kiba's near-death moment, where Satan's sister shouldn't have been there, raises questions and how she recruited other members of her peerage out like Issei and Akeno also suspicious as hell. Another factor contributing to her reputation as a betrayer is that other characters in her peerage, like Issei, Kiba, Ect. are extremely loyal to her to the point of fault which making them less suitable as a betrayers compared to Rias.