My idea is that whilst mining the stone at a very fast pace, the pickaxe will accidentally swing out of Steve's hand and ends up lodged in the Zombie's head, because of the momentum of the pickaxe, it sends the zombie tumbling into the lava pool, Steve walks to where the ladder was, stepping on the spider in the process. Realising that there is no ladder, Steve just starts punching the stone to create makeshift stairs, because it somehow works with trees..