Why do my tomatoes start to rot on the same fruit part?
Mold spores float around everywhere in the air, in your home and on farms. They are also likely found on the surfaces of farm equipment used for harvesting (shears, knives, etc.). Grey mold can infect tomato leaves and stems where there are lesions, such as those introduced when harvesting fruit, when cutting the fruit off at the stem. This could happen wherever such spores can find ingress between the farm, produce distributors, the market, and your home.

Cite: https://content.ces.ncsu.edu/botrytis-gray-mold-of-tomato