Why would a doctor who didn't initially recommend surgery for esophageal cancer change his mind?

Many possibilities . Any medical decision is not black or white. There are many variables that need considered. The doctor is trying to make the best decision for you based on guidelines, but to make the decision he or she have to relly on information gathered by many sources. Your physical exam, x-rays, invasive tests like esophagoscopies etc. As all information accumulates the decision may change accordingly.
Depends. The doctor may have seen something on the pet/ct scan or ct scan that may have showed response to chemotherapy +/- radiation. It may be best to get a second opinion from a surgeon certified by the american board of thoracic surgery, and who practices at a high volume academic medical center.