It could be a few things. The obvious one is if you've planted tomatoes in the same bed for a number of years.
If they've been planted in the same bed, then you may have nematodes. The normal non-toxic treatment, is to plant marigolds and mustard plants in it, let them mature until they're sappy, then dig them in - natural fumigant.
Otherwise, it may be inconsistent watering, wetting the foliage, fertilising at the wrong times, etc.