C-a <TAB> is bound to the "focus" command. "focus" can receive parameters ("up", "down", "top" and "bottom") Without parameters, it defaults to "down", which is why you cycle downwards with you press C-a <TAB>.
To tab backwards, you call "focus up". It's not bound by default, but you can bind to, say, C-a U:
To tab backwards, you call "focus up". It's not bound by default, but you can bind to, say, C-a U:
bind U focus up
and from then on "C-a U" cycles upwards.