Originally Posted by
tomato coupe
You tensed up, that's all.
This could be the culprit. Tensing up when the bus went by could do it. When I was racing my bike, a well maintained Peugeot PX10, would get into a wobble on descents, especially when riding with other riders surrounding me. We changed a lot of parts, including a Spidel roller bearing headset. What we finally found was that I was tensing my back and shoulders, and probably gripping the bars harder. Once I learned this and relaxed more it eliminated the wobble. My bike was a 62 cm, 531 Reynolds frame, and had a lot of flex.