It could be that it was exposed to humidity and/or corrosive salts were still present on the metal after the last cleaning.
There are a lot of good corrosion protection products but I don't know if they will defeat corrosive salts for very long.
Eezox Gun Care is one of the best protectants against moisture.
Many well stocked gun shops carry it for $5 or $6, or check eBay.
http://www.eezox.com/gun-care.html
http://www.eezox.com/