When moving air encounters an obstacle like a mountain, it is forced to rise up and over it. As the air spills over the other side, the pull of gravity causes it to overshoot a little before resurging back up. It's a bit like a car's suspension bouncing after hitting a speed bump.
A stable air mass will continue to rise and dip for a little while as it travels away from the mountain, setting up an invisible "standing wave".
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