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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:

bind U focus up

and from then on "C-a U" cycles upwards.