I have said for years that Letang smacks of a player who has simply gotten by at every single level of hockey on his skill alone. He hasn't had to try to be better because he just simply is. When you start playing against the best in the world for 20 mins a game though...your skill is great, but they will find ways to expose you. Letang is being exposed, partially.
The other part of his problem is that he just cannot help himself. He's like a puppy watching a biscuit and he has absolutely no impulse control. When he sees an opportunity to pinch, he just cannot help himself, where if he were a Lidstrom, he'd know that if he pinches, he's leaving his partner high and dry, and it is a situation that can turn ugly in a quick hurry. Part of being a truly excellent hockey player is being able to calculate risk on the fly. That's part of the evolution of a player. Letang needs to evolve his decision making, and right now, his decision making is piss poor at best.