I've flown in the area for over 50 years, That particular ridge is right above the Columbia River & there can be some horrific downdrafts there especially with a strong west wind. It's just east of Ellensburg & Ellensburg is famous for its strong surface winds. A 150 isn't know for having excess power. I'm real interested to hear the NTSB report on this one. When you get in a strong downdraft it's counter-intuitive to push forward for more airspeed but that's what you have to do. If you keep pulling back you will stall. I learned that while teaching in and flying gliders. RIP pilot.