Intense heat or light is the culprit. Ways to prevent it are having good leaf cover, having a good potassium level, and selecting varieties that are not prone to green shoulders. If all else fails pick the tomato as it first begins to ripen and put it in a window.
Here is a July overview picture of my garden. I like the way it looks now. Everything looks in its place and there's not nearly as many weeds as there was a month ago. Still waiting on my garden gate. I think it will be almost perfect then. The ear of corn that you can see in the picture was attacked by what I'm assuming was raccoons. They tore through the husk to get to the little kernels inside. That does not bode well for corn next year.
This is one of my Tender Pick bush beans. The beans are almost ready to pick. I was told to wait until they are 5 inches long. The "test" bean i picked today was 4.5 inches. Looks like another day or 2 before harvest.