Here it says Goren Kidon. In Divrei ha'Yamim I, 13:9, it says Goren Nachon!
Radak #1: It had two names.
Radak #2: (It is called Goren Nachon.) Here it says Goren Kidon, for Uza died there. Kidon refers to death, like "Yir'u Einav Kido" (Iyov 21:20).
Radak (Shmuel II, 6:6 citing Sotah 35b): First it was Kidon. Later it was Nachon. I.e. now when they carried the 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 (proper); the Leviyim carried it on the shoulder, like it says below (15:15). There it says (verse 26) that Hashem helped the Leviyim, who carried it properly via poles on the shoulder, in contrast to here, that He killed Uza.
Here it says "Le'echoz". In Shmuel II, 6:6 it says "va'Yochez"!
Malbim: Here it explains that he stretched out his hand [in order to hold it]; he died before he actually held it.
What is the meaning of "Shamtu ha'Bakar"?
Rashi (Shmuel II, 6:6, 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 (Shmuel II, 6:6): 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 (Shmuel II, 6:6): 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 (Shmuel II, 6:6): 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.
Malbim (Shmuel II, 6:3-5): There were six ways in which they did not show proper honor for the Aron (a) "Im Aron Elokim" (Shmuel II, 6:4) teaches that they [carried it from the house, and] held the Aron itself with their hands. It must be carried via its staves! (b) They carried it on a wagon, and not on the shoulder. (c) They were conducting the wagon. Even the Pelishtim knew to let the cows lead it by themselves! (d) Uza and Achyo carried it, and not Leviyim. (e) Achyo was in front of the Aron; his backside faced it, which is disgraceful. The Pelishtim went in back of it! (f) David and Yisrael were jovial in a lightheaded way, not happy amidst fear, and did not play the special instruments for Avodah, and not via the Leviyim.
