Why did David put the Aron on a wagon? Bnei Kehas did not get wagons, for "they carry the most Kodesh matters (including the Aron); they will carry on the shoulder" (Bamidbar 7:9)!
'Rashi': He said, it came to me when Pelishtim put it on a new wagon [and thought that also we may do so]. Therefore, he was punished.
Rashi (Shmuel II, 6:3, from Sotah 35a): Because David called Divrei Torah "Zemiros" (Tehilim 119:54), he was punished, and erred about something that even children [who learned only Chumash] know. Therefore, when he brought it [afterwards] from Beis Oved, they brought it on the shoulder. He told the Leviyim "Hiskadshu? Ki lemiba'Rishonah? Lo Derashnuhu ka'Mishpat" (below, 15:12, 13).
Radak (10): David thought that the verse applies only in the Midbar, when the Mishkan was carried via wagons, to show that the Aron is more Kadosh than the Mishkan, but now it is permitted on a wagon. It also came from Sedei Pelishtim on a wagon! He erred.
Vilna Gaon (15:2): He thought that the verse applies only in the Midbar. They were Dor ha'Midbar; all were Kedoshim, and Dor ha'De'ah. In David's generation, it is proper to carry it only via a wagon. However, after Hashem killed Uza, he said that only Leviyim should carry it ? forever.
Why was it a new wagon?
'Rashi': Refer to 13:7:1:1.
Radak: [It is proper to use for the Aron] something that a commoner never used. It was made for Kedushas ha'Aron.
Who are Uza and Achyo?
'Rashi': They are sons of Avinadav, like it says in Shmuel II, 6:3.
Were they conducting the wagon?
'Rashi': Yes, like is normally done ? one was in front and makes it go straight, and one is at the side, lest it flip over and fall. So it says in Shmuel II, 6:4 ? Achyo was in front, and Uza was at the side. Therefore, Uza stretched out his hand when Shamtu ha'Bakar.
Radak (Shmuel II, 6:3): They were conducting the cattle that were pulling the wagon.
