Fatigue isn't the only issue here.
He was an unknown commodity with no NHL experience when the season began. He scored a few goals early off, had a highlight reel shootout experience, and a few weeks later he's among the lead leaders in goals. Suddenly, teams stopped treating him like a soft Euro and started taking away his time and space. He's more tightly covered whilst sneaking into the slot from the soft-side. Defense are on him much quicker and harder in the corners, and while Brunner can mentally adjust to North American hockey, it will be a slower process to build up the physical strength to be able to fight through the coverage that he is now experiencing.
Secondly, Brunner has really focused hard on playing a more defensively sound game. He is back-checking much more effectively than when the season began, and he's playing much deeper in our own zone instead of floating at the blueline waiting for a breakout. He has also been more conscious of opponent's breakout potentials and is adjusting his game in the offensive zone to accommodate this. His new-found defensive responsibilities are also going to lower his offensive output.
Combined with the brutal North American schedule and travel, its no surprise at all why Brunner has suddenly gone quiet. He's with good role models though, and I suspect that he will be able to overcome the schedule and the added defensive attention he has over the next few years. Given his commitment to learning the defensive side of the game, and how far he has come already, I'd say the guy is willing to do whatever it takes to be successful at this level. Lets hope my assumption is correct.