http://www.chinesedrywall.com :
"In short, drywall imported from China which emits gases which corrode copper and metal surfaces, gives off a foul odor and can make you sick."
"In short, there are numerous theories and no consensus. The most common theory is that the tainted drywall was manufactured in gypsum mines in China which used fly ash, a waste material that is a byproduct from power plants using coal. Coal fly ash can leak into the air and emit sulfur compounds. Samples of Chinese drywall recently tested by United Engineering, however, consiste of 5-15% organic material, which contradicts the assumption that Chinese drywall was made of waste from coal fired power plants.
Another theory is that Chinese drywall contains bacteria which is degrading iron and sulfur compounds to produce sulfur odors. Drywall imported from China was kept on barges at sea for months awaiting permission to enter the United States. While at sea, the drywall was exposed to seawater. In fact, there are reports that the drywall was wet (and stunk) when unloaded from the ships. According to many experts, however, if bacteria is in fact present, it is not significant enough to cause an odor.
Testing of drywall outer paper and the gypsum core has been found to release sulfur compounds. Thus, there are potential causes aside from the drywall itself, including contaminants in the adhesive that binds the paper to the drywall. Additionally, the drywall or outer paper may have been treated with a fungicide after entering the United States. No one has yet to determine whether tainted drywall has been found in China or Germany (where Knauf is based). "