Cystic fibrosis pts are prone to excessive fluid loss while exercising cause Cl in their sweat cant be absorbed which attracts Na and water and cause dumping of lot of electrolytes and water in sweat resulting in dehydration.
In response to that kidney (PCT) absorbs more HCO3 than usual causing contraction alkalosis
Loss of significant amount (volume contraction) of HCO3 free and chloride containing fluid (sweat loss in CF patients) raises HCO3 in plasma, bicarb is now more than normal in small volume i.e it is dissolved in a smaller volume of fluid.
Volume contraction can be a cause of alkalosis too by activating RAAS:
Volume contraction -> decreased BP -> activation of RAAS -> increased AT II -> increased/stimulated Na/H echange in PCT -> increased HCO3 reabsorption -> alkalosis, due to volume contraction, aka Contraction alkalosis.