The major rumor, at least on the PT forum was from one of the most frequent (and most trusted) posters who had an unnamed source (close to the band) who said the PT break-up initially came from a small feud between Steven, and Richard/Gavin on the other side. Basically what this was all about was SW approaching the band with new material sometime after writing Grace, and Gavin/Richard not liking the direction. And if this is true, Richard and Gavin wanted to re-work the material drastically and have the whole band very involved in the process, and Steven just wasn't willing at all to let go of so much control, and that's really all up to him considering he has been the main driving force and songwriter for the band since the start, and it comes down to him in the end. But basically the "clash" in creativity and not agreeing on a direction was what caused the "rift", and that's when SW decided to make something bigger with his solo career, and he announced major tours for Grace shortly after, and has since only focused on his solo career.
Now, like everything you hear, do take this with a bit of salt. It's impossible to find out exactly what happened, and even if we heard Steven's story about it, that might not be the whole story either, so only they know. But what I do find interesting about this rumor is the fact that a lot of it makes sense. If we start with the creative side of the band, PT did have a period of time when it felt much like a band collaboration. SW was still the main guy, but from Signify to Deadwing, the band felt like a band where everybody had a lot to do with the sound. Fear of a Blank Planet and The Incident were different in that sense. It became more about SW, and while it had always been mainly about him, these two albums did sweep the other guys to the side a bit more than before. An argument over direction? Totally possible. SW has said it himself that the music he wrote for Grace wasn't the cup of tea for some of the PT members, and so it would be logical. Richard/Gavin possibly wanting to get a bit more involved songwriting wise would also make sense, who doesn't want a say in the music your band makes? So again, a grain of salt, but it's not THAT far fetched and a totally plausible explanation for why they went different roads after The Incident and put the future of PT on ice. If it was a bad or friendly decision, that's really the key part of it, because that would either open the door for a PT future, or close the door completely.
SW seems very happy with the current state of his career, and why wouldn't he? While us hardcore fans still talk about Porcupine Tree, he has done a great job of breaking free from those bonds. I remember when Insurgentes came out. Not that the album received bad reception or anything, but a lot of people were worried about PT, but with the release of Grace and especially Raven, you hear those whispers less frequently, and more people seem at peace with the idea of never getting another PT album, as long as SW keeps doing what he is doing. He has reached this "godfather of prog in the 21st century" status, and his popularity isn't really decreasing despite PT being out of the picture, if anything he seems to get bigger with each solo album. It really comes down to SW and his stance on how much control he wants to have. With his solo music he has all the freedom in the world. If the current guys doesn't like what he has in mind for the next album, they can go, and SW will still be SW. In PT, he would have to adjust the music a bit more to fit the vision of the other guys, and I don't think the possibility of just firing someone in PT, or splitting ways is as likely. PT wouldn't really feel like PT without Richard, Colin or Gavin. So in conclusion, SW doesn't really need to go back to PT, unless he really wants to. His solo career is doing great, but it would really be a dream if PT came back in big fashion. After Grace and Raven and how much SW has matured as a songwriter, I want to hear the 2015-2016 Steven Wilson and his vision, with Richard, Colin and Gavin by his side. Magic would happen.