The bash auto completion is broken in xfce terminal ; In the terminal app, tab is used to switch focus to another terminal ; completely incompatible with all other desktops ever produced that use alt-tab - and there is no way to change it either ;
Check $HOME//.config/xfce4/terminal/terminalrc for what "<Actions>/terminal-window/next-tab" is set to be - I don't know how it would have been changed to use <TAB>, but perhaps that was accidentally changed somehow. In a default configuration, what you describe does not happen.
*** Bug 10907 has been marked as a duplicate of this bug. ***
Probably, a misconfiguration issue.