The sulfoxidation process is responsible for converting sulfite (toxic) into sulfate (necessary). If sulfoxidation isn't working well, then it can get overloaded, resulting in a sentitivity to sulfur foods.
Sulfoxidation needs molybdenum and vitamin B6, but too much of either of these can inhibit the process. The ballpark for too much is around 50-100mg B6/day, or toxic-high levels of molybdenum. The most common culprit seems to be low molybdenum.
Reasons to Suspect
Asthma, eczema and other symptoms of low sulfate.
Sulfites are formed from sulfur foods, and are naturally occuring in some foods like wine. They are used as a preservative, and don't have to be labeled - common sources that I know of include dried fruit, shrimp, and processed corn. Feel free to send me a good link if you find one ;)