It's mentioned that he cave was found by reborn lackies father or grandfather. They used to come there from time to time so they stored all kinds of supplies and planted veggies. The reason lacky pushed him down is because he knew of the cave. He wanted to save MC back (as there is no path down. You can only get there by rope. Unfortunately he died and MC couldn't get help in time so he slowely went blind. I'm not sure if he as poisoned or it came from his injuries.