Another interpretation: "if a man opens" - to make him liable for this one on its own and for that one on its own.
Held Liable for Each Animal That Falls in the Pit
Yalkut Shimoni on Torah 341:10
דָּבָר אַחֵר, כִּי יִפְתַּח, לְחַיֵּב עַל זֶה בִּפְנֵי עַצְמוֹ וְעַל זֶה בִּפְנֵי עַצְמוֹ.