Why concrete sinks
Concrete sinks when the soil under it settles or washes out and leaves a void beneath the slab.
Disturbed, re-compacted soil is never as stable as undisturbed ground, and a slab that dropped quickly usually points to burrowing animals, misdirected downspouts or poor grading. We point out the cause when we see it, because leveled concrete can keep sinking if the soil beneath was never packed to "Proctor density", the trade's compaction standard.