Can’t believe it took so long for the obvious reason:

“The survival and growth of the bunker ‘colony’ through the years, without producing own offspring, was possible owing to continuous supply of new workers from the upper nest and accumulation of nestmate corpses,” the team concluded. “The corpses served as an inexhaustible source of food which substantially allowed survival of the ants trapped down in otherwise extremely unfavourable conditions.”

They kept falling in, and the ones that died got eaten by the ones who didn’t.

Once they could climb out, they all just climbed out as soon as they fell in.