Usually the cause is molds and yeast growth. Likely the canned wasn't properly sealed or processed. All it takes is one small hole to allow air in.

Dave

And if you buy sauerkraut, buy the stuff in the cooler section; it's a much better quality product.

Yeah, we usually do buy the refrigerated bags but always keep a few cans around for Ruebens, etc. The best sauerkraut is homemade but I didn't make any last year.