Another interpretation: It [the case of the goring ox] was included in the general rule [of capital cases], and it went out to be treated as a new matter, and [yet Scripture] returned it to its general category. "And its owner also shall be put to death" (Exodus 21:29) - [this means] at the hand of Heaven. Or perhaps it means only at the hand of man [by the court]? When it says, "if a ransom [is laid upon him]" (Exodus 21:30), [we learn that] ransom is given for those who are put to death at the hand of Heaven. Or perhaps it is only for those put to death at the hand of man? When it says, "you shall take no ransom for the life of a murderer" (Numbers 35:31), [we learn] that no ransom is given for those who are to be put to death at the hand of man.
The Negligent Owner Answers to Heaven, Not the Court
Yalkut Shimoni on Torah 340:10
דָּבָר אַחֵר, הָיָה בַּכְּלָל, וְיָצָא לִדּוֹן בַּדָּבָר הֶחָדָשׁ, וְהֶחֱזִירוֹ לִכְלָלוֹ. וְגַם בְּעָלָיו יוּמָת, בִּידֵי שָׁמַיִם. אוֹ אֵינוֹ אֶלָּא בִּידֵי אָדָם, כְּשֶׁהוּא אוֹמֵר אִם כֹּפֶר וְגוֹ', הֲוֵי נוֹתְנִין פִּדְיוֹן לַמּוּמָתִין בִּידֵי שָׁמָיִם. אוֹ אֵינוֹ אֶלָּא לַמּוּמָתִין בִּידֵי אָדָם, כְּשֶׁהוּא אוֹמֵר "וְלֹא תִקְחוּ כֹפֶר לְנֶפֶשׁ רֹצֵחַ", הָא אֵין נוֹתְנִין פִּדְיוֹן לַמּוּמָתִין בִּידֵי אָדָם.