I get this question quite frequently and there are a plethora of possible reasons for an uneven patio but here in Michigan, the most common reason is that the water in the ground under the patio freezes in the winter. The ground expands when it freezes. This ground expansion lifts the patio. This is called frost heaving. Of course, Some areas under the patio will hold more water than other areas which is why your patio will become uneven.
You might wonder “Do all paver patios heave and become uneven eventually?” The answer to this is a resounding NO! A properly installed patio will have a thick enough layer of compact crushed limestone (21aa) or clean limestone to protect the subsoil from the effects of the freezing (expanding) ground. Of course, there are many ways that installation can go wrong which can lead to pavers becoming uneven..
How can you be sure that your contractor will be installing a patio that will not heave over time? There are a lot of things to look for in a contractor but the easiest thing to look for is a warranty on the work. The industry standard warranty is 2 years and covers the heaving of the pavers among other things. If the contractor doesn’t offer a warranty for 2 or more years, that’s a red flag. Time to look for another contractor. Certifications such as ICPI. SRW and others can give you more peace of mind that work will be done correctly.