Thanks for writing. When cucumbers drop their fruits before the fruits mature, it usually means the plants are stressed --- it could be from too much or too little heat, or too much or too little water, or there's been a sudden change in conditions (like it goes from 70F one day to 95F the next).
Your cucumbers may also shed their fruits if the plants are bearing more fruits than they can support. If your plants have lost leaves to disease, for example, they may drop fruits to make up for the leaf loss.
---Justin, Senior Garden Editor, BHG.com