The first thing to note is that expectations were obviously too high for Leslie coming into the season. It was the same problem that got NC State ranked sixth in the nation - the press looked at a three-week span at the end of last season and extrapolated that out for his entire junior year. Sometimes this works - players take leaps forward and the development sticks and they become much better players. Sometimes its just a hot streak, and that appears to be what happened with Leslie.
It's not about being 'engaged' or 'energized' or whatever buzz word you want to fill in for why Leslie plays well sometimes and why he doesn't. Leslie is simply enigmatic and always has been. His problem this year is that he's actually gotten worse in many phases of the game - he's not the same defender or rebounder he was last season and its hurting State's defense substantially to not have that version of Leslie (as much as people slammed Leslie on defense last year he always rated as their best post defender - his natural athleticism allowed him to make plays even when it looked like he wasn't 'engaged').
Let's break this down a little deeper to see where Leslie has faltered.
So what we can see here is that while Leslie is actually shooting a little better (and getting to the line more often as well), he's gone backwards in many of the other things that made him a valuable presence on the team. The biggest thing is the turnovers, which have skyrocketed at the same time his assist rate has dropped. But he's also crashing the glass less, not blocking shots or getting steals like he used to - though I suppose if he could get those awful goaltends to count as blocks he'd make up some ground.
In general, he's just a worse player than a year ago. Which isn't to say that he can't repeat his performance in March of last year, its just that he didn't build off of that kind of hot streak last year and become a better player.