Scary, Too Scary
The earthworm-type demon beast clones dug through the rocky layer, mostly filling up the path they made behind them as they proceeded forward. This was to ensure the column they dug wouldn't collapse when they encountered something amiss.
Moreover, if a clone died after digging into a field of lava, it would cause the lava to flow through the tunnel it had dug, allowing it to flow through the entire rock section. That would restrict the paths Krune could move through during his escape.
There was also the possibility that if a similar thing constantly happened, before he could realize what was happening, he would be surrounded by tunnels of lava. After that, chances of him making a successful escape would become slim.
That was why all the clones were careful in their movements, tunneling through while filling up the parts they had dug immediately. This slowed down their exploration speed tremendously, but it was the safest approach.