Why it happens
Each category gets a fixed number of visas per year, and each country is capped at a share of them. When more people with early priority dates apply than the State Department expected, it moves the cutoff back to keep use within the annual limit. Retrogression often shows up late in the fiscal year (summer) as numbers run out. The fiscal year starts October 1, when new numbers become available.
What it means for a pending case
If your date stops being current, your case generally cannot be approved until it becomes current again. A pending I-485 usually stays pending rather than being denied for that reason. Ask your attorney how it affects your case.