I have no research to back this up, but I think it’s just a glitch that isn’t detrimental to passing on one’s genes, so the trait got to stay around. Not everyone has it.

But if I were to hypothesize an evolutionary benefit, maybe it is a good thing to sneeze out all the fungal spores from your cave once you emerge. But then again, if it was important, shouldn’t this also induce coughing?