Nes, I can't say for sure either, but some of my green ones are doing this too... even the ones that I soaked in baking soda solution, hoping to kill any surface fungi. But many if not most of my tomatoes are salvaged from plants with signs of Late Blight. If it's IN the tomato, I think you just can't do anything about it. Big toms like the ones you have, I would cut out the brown parts and use them for some kind of cooked green tomato dish. So far, I've had fried green toms, green tomato and sweet or jalepneno salsa, and curried green tomatoes. I can recommend all of them.
I've noticed that even in a cardboard box of assorted tomatoes with browning tomatoes, SOME stay fine and others turn. (In fact I have to sort through them today, haven't done it in about 3 days. So there's attrition but some have continued to ripen without damage.
I have my really "valuable" tomatoes lined up upside-down on cake/cookie cooling racks for better circulation. You might want to try that. They end up with rack dents on the bottom (like grill marks
) but they seem to hold up much better. Be sure to check every day because they seem to manage to go bad from one day to the next.