Mountain Lake Wind Lines

You can see the pressure above the red motor boat if you tilt your monitor back and forth

At the Mobility Cup 2014 at Lower Kananaskis Lake, I was noticing that the wind was coming down the valley/lake in very defined waves of pressure.  They made for very good racing at first when the bands of wind came at a frequency that let sailors see them from a distance and get to the appropriate side of the course in time to take advantage of the pressure.  But as the day wore on and the breeze lightened up it made for fluky sailing.  What happened was that sailors could see a band of wind coming and head over towards it, even get into it, but then that wind would quickly die and the next band of wind would come before the boats had time to change their position on the course.  So if the new wind favoured the other side, tough luck.  In a couple of situations the people who had bailed out and for whatever reason had not gone for the pressure ended up in the next, more substantial wave of pressure which had not been visible on the horizon when they first bailed out, it was effectively an upwind sucker gust.

Does anyone know why mountain lakes get these very well-defined bands of pressure rolling through particularly in a dying breeze?  I also remember this happening on  Lake Waterton the few times we have had training camps there and of course having it happen on 2 lakes makes it a rule - kidding.

This one is a blast from the past, Lake Waterton: I think I see a wind line to windward of 178268