From your photo it looks like the tomatoes were subject to one or more foliar diseases such as early blight and/or septoria, and gray leaf spot. All produce spots or lesions on leaves, start at the bottom of the plant and progress through the season if conditions are right, to defoliate tomato plants (spotting, yellowing, dying.)
You can remove the lower leaves to slow the upward spread of the disease, keep plants well mulched to minimize soil splashing, water at the base of the plants, provide adequate spacing and spray with a copper fungicide (early in the season) to lessen spread of the disease. It is too late to spray this season. Fungicide treatments can help to protect new or un-infected foliage they are not curatives. See our website for more information on management of the diseases. http://extension.umd.edu/learn/early-blight-tomato
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