Another interpretation: if it fell, one returns and rebuilds it. Rather, just as a fist is distinctive in that it can be observed by witnesses, so too anything that can be observed by witnesses [is included]; this excludes the case where a stone flew from beneath the hand of the one who struck, in which case he is exempt.
What the Fist Teaches About Liability for a Blow
Yalkut Shimoni on Torah 331:4
דָּבָר אַחֵר, נָפְלָה חוֹזֵר וּבֹנֶה אוֹתָהּ, אֶלָּא מַה אֶגְרֹף מְיֻחָד שֶׁמָּסוּר לְעֵדִים, אַף כָּל דָּבָר שֶׁמָּסוּר לְעֵדִים, פְּרָט לְשֶׁיָּצְתָה אֶבֶן מִתַּחַת יָדוֹ שֶׁל מַכֶּה, פָּטוּר.