The fatal accident was not in a lava tube. It was in one of the "Gallerias" (I think the spelling is correct) which are man made tunnels driven into the heart of the island in order to extract water. The entrance to this particular Galleria is in a lava tube, but the deaths occurred a long way into the man made tunnel.
It should be noted that as the Gallerias are driven into an active volcano, they are renowned for pockets of nasty volcanic gases building up in odd places.