shimmy
My guess would be too light-built a front rack, not the weight itself. Most of my tours have been through remote mountainous or hilly terrain, meaning I carry a lot of weight, both front and back, and hit very high speeds downhill. My front rack is a converted rear rack mounted above the front wheel, and my panniers are mounted high, similar to the rear panniers. It is a well-secured, rigid rack. I've never experienced any shimmy or wobble. I am a heavy person, so I have to place a great deal of the total pannier weight in the front panniers in order not to over-load the rear axle and tire.