Does anyone know what causes bitter cucumbers? So far I've harvested a dozen and almost all of them have been bitter. They are still edible but I would like to fix this bitterness.

maybe you harvested too early

I have read that if you have a hybrid/all female cucumber plant, it can form male flowers when stressed. If these male flowers pollinate your female flowers, it can make the fruit become bitter.

check your seed package. If it says they are Hybrid/F1 seeds... then pick off all male flowers that form. You will still get cucumbers; however, they shouldn't be bitter.