Probably got a little moisture in the sheath along with dirt. I've had mine for 12+ years, bought it directly from George Lesche when he still owned the company and it has had a little surface rust on it at times and even a little discoloration. I usually spray it off when I get home, dry it and spray a light coat of WD40 on it and my shovel.
Take decent care of it and it'll last for years.