Quoting Commentary for II Samuel 6:7

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.

I will go mad Heb. שׁוֹלָל, becoming mad like a mentally deranged person, with bewilderment. Cf. (Ps. 76:6) “The stout-hearted have become mad (אֱשְׁתּוֹלְלוּ)”; (Isa. 59:15) “And he who turns away from evil is considered mad (מִשְׁתּוֹלֵל).” The ‘tav’ comes into the word to serve in the reflexive present and past. This is the normal pattern of a word whose first radical letter is ‘shin’ or ‘sammech’ when a ‘tav’ is added in the reflexive, either present or past, as in מִשְׁתּוֹלֵל. I say that it is an expression of error, as in (II Kings 4:28) “Do not mislead (תַשְׁלֶה) me”; (II Sam. 6:7) “For his error (הַשַּׁל) ;” (Dan. 3: 29) “who speaks anything amiss (שָׁלוּ).”

and He struck him down because he stretched forth his hand upon the Ark This is what is written in II Samuel (6:7): “... and God struck him down there” עַל הַשַּׁל meaning עַל הַשֶּׁלַח, because he stretched forth to hold the Ark. Another example of this is (I Kings 21:23): “The dogs will devour Jezebel” בְּחֵל יִזְרְעֶאל. In another verse it is written: (II Kings 9:10): “בְּחֵלֶק יִזְרְעֶאל,” in the territory of Jezreel. So will we explain עַל הַשַּׁל here as הַשֶּׁלַח

and he died there before God before the Ark of God, and in (II Samuel 6:7): “...and there he died by the Ark of God.”

is considered mad Heb. מִשְׁתּוֹלֵל, considered mad by the people. Comp. (Micah 1:8) “I will go as a mad man (שׁוֹלָל).” This is equivalent to שׁוֹגֵג, inadvertent, which the Targum renders: שָׁלוּ. Likewise (II Sam. 6:7), “For his error (הַשַּׁל).”

And let [some sheaves] fall for her. שֹׁל תָּשֹׁלּוּ =] you should forget; pretend as though you are forgetting.¹⁴The double verb indicates that the action was to be repeated many times. The Targum for שְׁגָגָה [=oversight], is שְׁלוּתָא,¹⁵Bamidbar 15:25. and similarly, “because of this error.”¹⁶II Shmuel 6:7. Another explanation [of שֹׁל תָּשֹׁלּוּ is “let fall”], an expression [as in,] “your olive tree will cast off [= יִשַּׁל [its fruit].”¹⁷Devarim 28:40.

The soles of the feet of the Kohanim were lifted. [They were lifted] from the water to the dry land next to them, and the water returned to their place. The result of this was that the Ark was on this [eastern] side and the Bnei Yisroel were on this [the western] side. Thus the Ark lifted its carriers and crossed over. It was regarding this that Uzzah was punished ‘when he took hold of the Ark,’⁷II Shmuel, VI: 6,7. for if the Ark carried its carriers, it certainly was able to carry itself.⁸Thus by taking hold of the Ark, Uzzah manifested his fear that the Ark was going to fall off the wagon.