Pressure
The dirt under a cliff is going to be under a lot of pressure.
Imagine being down there with all that dirt. Imagine feeling the pressure closing in around you. It's so hard to breathe... h... help!
あつ
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The dirt being crushed under this pressure can't help but react by sneezing out, "Achoo (あつ)!" This sneeze-inducing pressure probably has something to do with where earthquakes come from.
Imagine you're the dirt being pressed from every direction, the pressure so strong, and it squeezes an ACHOO right out of you.