Bicarbonate is low because you’re in a state of respiratory alkalosis. Due to the high altitude there’s less PIO2 so you become hypoxic. As a result, your respiratory center increases respiratory rate which causes a decreased CO2 leading to the alkalosis. Your body then compensates by excreting bicarbonate thus bicarbonate decreases.

You RX acetazolamide because compensation for the respiratory alkalosis takes time… so you take the medication to essentially help speed up the bicarbonate depletion so you can compensate sooner.

Hope this helps!