Another interpretation: "if a man opens" - I have only one who opens; from where do I include one who digs? The verse teaches, "or if he digs." So that one should not say: I have a claim from logic - if the one who opens is liable, the one who digs should not be liable? Now if you had said so, you would have imposed a penalty by logic. Therefore it is said, "if he digs," to teach you that we do not impose a monetary penalty by logic.
The Digger and the Opener and Why Logic Cannot Impose a Fine
Yalkut Shimoni on Torah 341:11
דָּבָר אַחֵר, כִּי יִפְתַּח, אֵין לִי אֶלָּא פּוֹתֵחַ, כּוֹרֶה מִנָּיִן, תַּלְמוּד לוֹמַר אוֹ כִּי יִכְרֶה. עַד שֶׁלֹּא יֹאמַר יֵשׁ לִי בְּדִין, אִם הַפּוֹתֵחַ חַיָּב, הַכּוֹרֶה לֹא יְהֵא חַיָּב. הָא אִם אָמַרְתָּ כֵּן, עָנַשְׁתָּ מִן הַדִּין, לְכָךְ נֶאֱמַר כִּי יִכְרֶה, לְלַמֶּדְךָ שֶׁאֵין עוֹנְשִׁין מָמוֹן מִן הַדִּין.