Go here: http://en.wikipedia.org/wiki/Ascorbic_acid and here: http://en.wikipedia.org/wiki/Sodium_ascorbate to see the structures of AA and Sodium Ascorbate. Your equation is not correct, as you cannot get free Boron and free Oxygen from this reaction.
Sodium Metaborate + AA = Sodium Ascorbate and Boric Acid which lowers the pH. By lowering the pH, if there is enough Borate or Metaborate and if the pH is high enough, the equillibrium shifts and the buffering ability of the ingredients kick in to maintain the pH as much as possible. That is a simple explanation of what is going on.
And, BTW, Borates are banned in many US Citrus states as Borates are toxic to Citrus trees.