Here it says that Goren Nachon. In Divrei ha'Yamim I, 13:9, it says Goren Kidon!
Radak #1: It had two names.
Radak #2: It is called Goren Nachon. There it says Goren Kidon, for Uza died there. Kidon refers to death, like "Yir'u Einav Kido" (Iyov 21:20).
Radak citing Sotah 35b: First it was Kidon. Later it was Nachon. I.e. now when they cadt h Aron on a wagon and Hashem killed Uza, it was Kidon (like a spear). Later, when David brought it from Oved Edom, it was Nachon; the Leviyim carried it on the shoulder, like it says in Divrei ha'Yamim I, 15:15. There it says (verse 26) that Hashem helped the Leviyim, who carried it properly via poles the shoulder, in contrast to here, that He killed Uza.
Did Uza send the Aron?!
Radak: He stretched out his hand.
What is the meaning of "Shamtu h'Bakar"?
Rashi citing Targum Yonasan: Meraguhi Toraya (Radak - the cattle dropped it. This is Po'el Yotzei (acting on others).) This is like "v'Nidchah Yado va'Garzen" (Devarim 19:5); the Targum is v'Sismerig Yede b'Farzela.
Radak: This is Po'el Omed (the cattle themselves were displaced), as if it said Nishmetu ha'Bakar. I.e. their limbs came apart due to the Kedushah of the Aron, which should not be carried on a wagon via cattle, rather, on the Leviyim's shoulders.
Malbim: They slipped out from under the Aron.
When the Pelishtim sent the Aron via cows, it went fine. Why was there a mishap now?
Radak: They did not know how it should be carried; they intended for the best way they could. They took a new wagon and cows that never has a yoke on them. Yisrael knew that it must be carried "on the shoulder" (Bamidbar 7:9); they sinned. Hashem showed this via the mishap with the cattle, and Uza's death.
