It may be caused by the use of excess nitrogen fertilizers or dry windy conditions, but the most common cause is temperature extremes. Tomatoes are especially picky about the air temps when it comes time to set fruit. If the night temps fall below 55 or rise above 75 or if the day temps are above 90, the pollen becomes tacky and non-viable. Pollination cannot occur. If the bloom isn't pollinated, the bloom dies and falls off.
Water the plants deeply once a week, mulch heavily to maintain constant soil moisture levels, establish windbreaks as needed, avoid using excessive amounts of nitrogen fertilizers, and wait for temperatures to moderate and stabilize.