Why does acid back up into the esophagus?

Weak sphincter. The sphincter between the esophagus and stomach can become weak or ineffective, causing acid to back up or reflux into the esophagus from the stomach.
See below. Gastroesophageal reflux occurs when the lower esophageal sphincter is either incompetent or it relaxes at inappropriate times. A hiatal hernia can be a contributing factor as can medications, caffeine, chocolate, peppermint, spicy foods and nicotine.