It seems incongruous for someone who has chosen such a dangerous vocation to have such a weak constitution when it comes to falls. We imagine spelunkers as rugged types who are used to braving inhospitable conditions in their zeal for cave exploration. Why would someone so tough not be able to survive a little tumble?
To answer this question, we must consider the nature of the dangerous job (spelunking) taken on by the blue-haired hero. It's totally subterranean. This is a job whose venues are located not only near the ground, but under the ground. You start low and only go lower. Just the thought of the unfamiliar, perilous, rocky environment would justifiably -- reasonably -- trigger all manner of worries, with one exception. In other words, our spelunker is terrified of heights.
When he falls from any distance, he is frightened to death before he even hits the ground. Our spelunker tragically underestimated the amount of vertical space inside the cave environment: by descending into a cave, he found a completely new layer of heights to torture him.