וַיִּשָּׂאֻ֗הוּ מִבֵּ֤ית אֲבִֽינָדָב֙ אֲשֶׁ֣ר בַּגִּבְעָ֔ה עִ֖ם אֲר֣וֹן הָאֱלֹהִ֑ים וְאַחְי֕וֹ הֹלֵ֖ךְ לִפְנֵ֥י הָאָרֽוֹן׃
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.
Rashi
drove the cart as is the custom of cart drivers, one walks in front of the cart and directs it in a straight way, and one is alongside the cart, so that it does not turn over and fall. And so it is written (II Sam. 6: 4): “... and Ahio went before the Ark,” and Uzza was alongside the cart; therefore, Uzza stretched forth his hand to take hold of the Ark when the oxen swayed it, and therefore, he was punished.