What went wrong?
From the looks of it, I'd say these are growth cracks caused by wet weather (or overwatering) followed by a dry spell. Irregular watering, overfertilization, and lack of calcium in the soil can also cause these types of growth cracks as they disrupt the ability of the plant to grow uniformly.
Consider adding calcium to the soil before you plant tomatoes and try to avoid adding too much nitrogen fertilizer. In addition, mulch will help maintain soil moisture and the fluctuation that can occur between watering and hot weather.
Here are some guides which may help prevent this problem in the future:
I hope this information is helpful and wish you continued luck with your gardening!