The main reason your driveway is sinking is because the soil under your concrete isn’t strong enough to support its weight anymore. This could be due to multiple factors.
The soil is eroding – If your property is sloped toward your driveway, water could be washing the soil out from under your concrete. This can cause it to sink and become unlevel.
The soil wasn’t appropriately compacted – This particular issue might date back before your driveway was even poured. Properly compacted soil prevents voids from forming in the dirt under your driveway. If your driveway installers skipped this process or didn’t do a very good job, your concrete might sink over time.
The soil under your slab is expansive – Expansive soil shrinks and grows based on how much water saturates the dirt. The more it swells and shrinks, the more your driveway will become unlevel.
The soil is filled with tree roots – If tree roots start infesting the soil underneath your driveway, they can cause loads of problems, including sinkage. If the roots absorb all the water in the soil, they can cause voids to form. If the roots begin to die, they can disintegrate and leave behind pockets, which will cause your driveway to sink.