Individual pavers sink over time as the base underneath settles or erodes — usually from water pooling in one spot, poor original base prep, or just years of freeze-thaw cycling. The fix isn't just resanding around a sunken paver; it needs to be lifted, the base built back up and compacted properly, and reset level with its neighbours.
Left alone, sunken pavers become trip hazards and create low spots where water pools instead of draining — which then accelerates the same erosion that caused the sinking in the first place.
Usually poor original base compaction, water pooling and eroding the base underneath, or natural settling over many years — crews can usually tell you the likely cause during assessment.
Proper base rebuild and compaction significantly reduces the chance, but if there's an underlying drainage issue nearby (like a downspout draining onto the area), that's worth addressing too.
Spot repairs of a few pavers are usually quick, done within a few hours. Larger sunken areas take longer since more base work is involved.
Yes — many people fix leveling issues at the same time as a full clean/resand/seal refresh.