Taiyang and Summer were both members at beacon of team STRQ. It is unknown what kind of relationship they had back then but they eventually fell in love. They then proceeded to marry and have Yang shortly after. It could also be inferred that since they were teammates, they might have been friends that turned to lovers. Taiyang seems to speak both fondly and negatively about Raven to Yang.
Yang tells Blake in volume 2 that Raven left Taiyang shortly after she was born, for some unspecified reason. This greatly devastated Taiyang as he was now a single father and wallowed briefly in his grief. In volume 4, Taiyang reveals to Yang that Raven was a very troubled and complicated woman, but that she always fought for what she believed in whether it was her team or her tribe. He then tells Yang that she has her mom's stubbornness. He then tells her that he's proud that she reminds him of Raven.
In volume 5 Raven calls Taiyang a fool infront of Weiss and Yang. It was also revealed in volume 5 that Raven's semblance allowed her to make a portal to someone she has a close connection to. Taiyang is then shown to be one of just 4 people she still has a connection to. In volume 5, chapter 14 as Tai is watering his plants he sees a bird feather fall near him. He then turns around looks up and glares at Raven.
Phoenix is a semi-popular pairing within RWBY. Since it was one of the few canon couples in the show. A lot of people think that even though not together anymore Raven still cares for Tai since she still seems to check up on him and Yang every once in a while. People also think Maybe him and Raven had a talk at the end of volume 5. It has a small fanbase on tumblr.
- "Phoenix" comes from the fact that Raven is named after a bird, and Phoenix's are a type of mythical bird.