There are a number of potential causes for this including the wrong soil type, too much nitrogen and extremes of either wet or dry weather.
Tomatoes don’t really like limey or sandy soil so if the ground you are using has this constituency you are either going to have to use containers or be extra vigilant when preparing the soil. A good idea for the latter is to dig a trench, line the bottom with newspaper and then fill it back in with a good well rotted compost and soil mix, potentially getting soil from another source if yours really is very sandy or full of lime.
If you think that perhaps you have overfed the plants with nitrogen then just stop feeding for a while and if you have put down a high nitrogen mulch, like coffee grounds, you can scrape it back off and try to eradicate the source.
A scenario called variable soil moisture has also been known to cause this condition, which is why it is a good idea to perform consistent and regular watering of the plants to establish a situation where the soil is neither saturated nor ever dries out.
With a little extra attention to your preparation and ongoing care it is relatively easy to avoid blossom end rot.