A little interview with Scott Taylor with Playstation had this info in it:
Playstation: This feels like a pretty big shift for Uldren — in the first game we knew he was kind of a jerk, then he kind of disappeared after the events of The Taken King… now he’s made quite the reappearance. How long has the team been planning for this to be where he returns?
ST: We’ve been working on this direction with Uldren for awhile. I don’t want to spoil anything, but there’s more going on with him than meets the eye. He’s the hero of his own story. You learn a lot about what’s motivating Uldren, and where he’s been, as you play Forsaken.
I like the fact that this story is going somewhere with Uldren and that it will be linked directly into the endgame raid and location. For all we know they could've took a leaf out of Marvel and realised that they needed to make a seriously good baddie that people hate but also really dig at the same time.