Harden is not choking...
Morey is praised for his genius in advanced metrics and building a team capable of accomplishing his philosophy. His philosophy of heavily emphasizing transition, in-the-paint and 3 point shots, thus eliminating mid-range shots on offense, is excellent for the regular season against sub-par, weary, and/or unprepared teams...
What happens in the playoffs?
Transition opportunities decrease as the pace slows and top 16 teams are better prepared to contain this type of offense...
When Harden has the ball at the top of the key, the Trail Blazers are playing a hybrid man and 1-4 zone, where they're trying to keep him out of the lane, but staying close to the shooters to challenge any 3 point shots.
Harden is a great offensive player for the Rockets' offensive philosophy (he's one of the best at getting to the rim, finishing in-the-paint, drawing fouls, and at contested 3 pointers), but he and the Rockets' philosophy are easier to contain on offense in the playoffs especially when Harden doesn't get fouls called.
The fact that Harden is one of the worst defenders in the NBA is magnified when he's off offensively. He's just terrible at it... almost a cone out there... but when he's on his game offensively, he counterbalance it, but when he's not, it becomes a problem.
To be a Championship caliber team in the NBA, you need a mid-range specialist that can create his own offense. LaMarcus Aldridge dominating predominately from mid-range must be a slap in the face to the Rockets and their philosophy.