Charles Ellicott Commentary 2 Samuel 6:3

"And they set the ark of God upon a new cart, and brought it out of the house of Abinadab that was in the hill: and Uzzah and Ahio, the sons of Abinadab, drove the new cart." — 2 Samuel 6:3 (ASV)

Upon a new cart. —The new cart, one which had been used for no other purpose, was undoubtedly intended as a mark of respect (compare 1 Samuel 6:7); yet it was a violation of the law (Numbers 7:9), requiring that the ark should be borne by the Levites. It is not necessary to suppose that David intended to violate the law; but since the ark had been left neglected for more than two generations, the exact requirements concerning it may easily have passed out of mind.

Abinadab that was in Gibeah. —Rather, in the hill, as the same word is translated in 1 Samuel 7:1. Abinadab himself may have been long since dead, and Uzzah and Ahio may have been either his sons, now advanced in life, or his grandsons.