Hey Ted,
I find that while wearing a tank, the pleats tend to swing side to side. Of course, I only see the end of the swing that I catch when it swings out to the right or the left. Wool has an interesting quality in that with heat and pressure it takes a crease and holds it. While the swing is in progress, the pleats unfold and allow the material to swing sideways. If you don't continue to add momentum, side to side by your walking stride, they return to the original position as pressed.

I'm an engineer but not a textile engineer and this is only my observation.