Sidewalks can rise or sink due to a few things, including but not limited to:
Improper soil compaction – Soil compaction increases the soil’s density, making it less prone to absorbing moisture and settling. If a contractor does not complete this critical process of soil compaction, the sidewalk may sink into the ground.
Poor drainage – During heavy rains, water could create channels that flow underneath the sidewalk, causing the soil’s structural integrity to fail. Check if you have a downspout from your gutters pouring in the direction of your sidewalk.
Local soil quality – Some soil sites are in high water tables, which receive more water than they can drain off. The unpredictable soil could cause the sidewalk slab to settle and sink.
Tree roots – A lot of sidewalks are parallel to streets that usually have trees planted along with them. These trees have large roots that could lift the slab, creating an ugly trip hazard. Unfortunately, when a root lifts the slab over 2 inches, the sidewalk section likely has to be torn out and re-poured, and possibly the root
Frost heaving – Frost heaving is when the ground freezes, concrete shifts out of place, and the slab rises as the soil swells. Then, when the ice thaws, the slab sinks. This process can repeat multiple times causing uneven sidewalks and cracks.