‘ And they brought it out of the house of Abinadab, which was in the hill, with the ark of God, and Ahio went before the ark.'
Once the journey had commenced Ahio went before the Ark as a kind of herald and forerunner, in order to prepare people for the coming of that sacred object. It was necessary to give advanced warning for all had to beware lest in some way they come in contact with or desecrate the Ark, and he may well at the same time have spoken out aloud concerning the glory of God (note Numbers 10:35-36). Compare how David would later dance before the Ark.
Thus it was left to Uzzah to drive the cart. This is important in the context of the story because it demonstrates that his attention was on the cart and the oxen. While all the others were looking at the covered Ark, he was watching the oxen. Thus when the oxen stumbled his first concern would be the control of the oxen. What had happened to the Ark would only come to his perceptions once he was sure that he had control of the cart.