Trucks elbow aside buses, which elbow aside cars, which elbow aside motorbikes, which elbow aside bicycles, which elbow aside pedestrians, because they’re bigger.
But I’ve heard a folk etymology explanation that reaches deep into Chinese history. When horsemen came through a village, the crowds of village folk, the 老百姓 or laobaixing, were supposed to part to let the rider through. When military or imperial processions were on the road, the horsemen had to make way. In general, whoever had the biggest, fastest, most powerful device for getting someplace could assume that smaller, slower, weaker travelers would get out of the way. This is not strictly a Chinese phenomenon: We’ve all seen the Wild West movies in which a posse thunders into town and the women-folk and onlookers scatter. Which I suppose is a reason to doubt the Chinese-specific folk etymology. But it makes me feel slightly less bilious toward the bus drivers. Instead of thinking as they bear down on me, “this is the way I will die,” I can tell myself, “these are heirs to imperial troopers.”