Answer: You are right, your plants were forming more roots, indicating that they needed to be planted deeper in order to have sufficient roots as the plant grew larger. Tomatoes are different from many garden vegetable plants in that they can form roots all along the stem. So when planting tomatoes, the usual recommendation is to remove the lower set of leaves and transplant it at least several inches deeper than it was originally growing (depending on the size of the plant you have). Roots will form all along the extra stem that is put underground, and this will give the tomato more support and the opportunity to better satisfy its water and nutritional needs.
If this happens during the summer again, just mound additional dirt up around the stem of the plant. It is not quite as effective as planting it deeper to begin with, but it helps.
Q&A Library Searching Tips