Tomato leaves turning brown can be caused by multiple factors including diseases, environmental stress, pests, and nutrient deficiencies. Common reasons include fungal diseases like early blight characterized by brown or black lesions with concentric circles on lower leaves, bacterial infections such as canker causing brown crunchy leaf edges, and pest infestations like spider mites causing dry, yellow, or brown leaves. Environmental issues like underwatering, leaf scorch from sunburn, frost damage, and nutrient deficiencies (phosphorus, potassium, calcium) also lead to browning leaves. Proper watering, disease management, pest control, and nutrient supplementation are key to addressing these problems.