I've been having the same issue on and off for the past couple of years. Only really shows up boarding, but feels like I've slightly hyper extended my knee. Gets better as the season progresses. Couldn't, for the life of me, figure out why, until this season
I sit watching TV with legs out straight and my heels on the coffee table. I always cross my left leg on top of my right. The weight stretches out my right (rear) knee. Football playoffs require a lot of couch time.
I could have hit the gym, but instead I bought an ottoman...problem solved.