Injuries and i know you asked for something more specific than just generally "losing a step"...but that's what it ultimately came down to.
To elaborate on that, Subban's game was always built around raw explosiveness. Everything he did well was about explosive speed and power. He never played a very efficient, cerebral game that was suited to aging gracefully. Some defencemen can afford to slow down because they're savvy positionally and understand how to get themselves to the right spots. Subban was always more...brute force, dynamic movement, and high effort to get himself to where he needed to be. His defensive play had a lot of explosive and reactionary elements to it...where he'd make plays defensively by sheer effort and raw athletic talent, rather than quietly making efficient plays proactively. There was a physicality to his game that would often carry him out of position on riskier hits and direct confrontation, but he had the athletic tools to make those sort of plays stick, and to get back into the play even when it didn't. Right up until he no longer did have the dynamic mobility to get away with that style of play anymore.
Even his play with the puck, he leaned a lot on just winding up and lugging the puck up ice himself. He was never really all that partial to the quick, efficient outlet pass to move the puck early easy. So as he started to lose a step, he started getting caught in bad turnovers more and more often, getting his pocket picked, etc. Which, seemed to spur him to also start trying to be more aggressive in making those risky stretch passes, which he was never the greatest at, and by that point...he just didn't have the mobility to recover when those things went wrong.
But long story short...he just started to really slow down and lose the dynamic explosiveness that his game was built on. And he just didn't play the sort of game that was made to cope with that loss of athleticism through efficiency and economy of action.