Rick is correct with low pH or acidic water but you can also get this from town supplies. When water is first introduced into the pipes it forms a protective layer or pertina. If this layer is not formed (low pH) then the acidic water accelerates corrosion by creating hydrogen ions.
In New Zealand we get this problem when some water authorities dose the water supply (Chlorine & Flouride). Unfortunately your problem is likely to continue getting worse. Get the water tested and if the water condition is going to be a problem then investigate the “plastic” pipes systems to replace the copper.