Titans showrunner Greg Walker has confirmed that the original season 1 finale turned into the season 2 premiere. There's long been evidence that the show's focus changed during production; when DC first announced the Doom Patrol cameo, for example, they told audiences it would happen in episode 5. Instead, it happened in episode 4; that change may well explain why Beast Boy was largely absent from the first few episodes, perhaps with some of his scenes cut.
The Titans finale also felt like a change of plans. Where most season finales are explosive stories that give every character a chance to shine, this episode was centered entirely upon Dick Grayson. None of the main actors were even onscreen with one another at the same time, perhaps indicating that this was a completely new episode stitched together to fill in the gap. It was presumed that the showrunners changed plans, deciding to resolve the Trigon plot in season 2.
That's now been confirmed by showrunner Greg Walker. In an interview with TV Line, he admitted that Titans season 1 was originally intended to have 12 episodes. According to Walker, a version of the original finale was cannibalized into the season 2 premiere. "We thought it was such a good cliffhanger at the end of 11," he explained, "and we wanted to go for an even bigger, better Season 2 opener. We had a big idea, and our friends at DC bought it."
Unusually, Titans was officially renewed for the DC Universe streaming service before the first episode had even aired. This seems to have given the production team the confidence to switch things up, and according to Walker the season 2 premiere will now be used to launch the new villain. "Because Trigon’s role in canon is so huge," he added, "we felt like we really needed to give him a really big episode, the kind you can launch a season with."
While this decision is understandable, it hasn't made for a particularly satisfying first season. The plot has taken numerous side-steps, with almost half the episodes feeling tangential to the main story - ostensibly the fall of Raven and the arrival of Trigon. In the end, the season 1 finale didn't even advance the plot all that much; the bulk of the story took place in a fantasy dream-world created by Trigon in order to corrupt Dick Grayson, with only the final two minutes set in the real world. That's a very unusual approach for a season finale, which is usually used to utterly transform a show's status quo for all stars.
Although Walker didn't officially confirm it, his responses suggested that the season 2 premiere will at least resolve one abiding mystery; just why has Raven sent Hawk and Dove on a mission to recruit Jason Todd? It looks as though Trigon will find himself opposed by a larger group of superheroes in season 2, with the not-quite-Titans consisting of Donna Troy, Starfire, Hawk, Dove, Jason Todd's Robin, and presumably Superboy and Krypto the Superdog. Trigon may have turned Robin, but he's still got his work cut out for him if he really does aim to conquer the Earth.
Source: TV Line