If the account was shared or hacked (even shared with a friend or guildie), then everything makes perfect sense. The (guildie) thinks:
"hey this person hasn't been on for several weeks ... let's make room for the guild. Oh hey I have the login too!! Hmmmmm"
*sell gear for coins*
*smack*
*smack*
*smack*
"He won't mind, after all he quit the game!"