I don't see this very often but on this particular tomato plant I've had a few of these now. And I believe the culprit is excess moisture. This plant grows in a self watering container made from a 5 gallon bucket but the fill hole is on a side I can no longer access. So I started watering it from the top and as a result it's probably not getting a consistent supply of water regularly.
There are more tomatoes on the plant that are not yet affected so maybe I should get out there and fix this watering problem. Another possibility is low calcium levels and I think I have some calcium suppliment I can use. But the soil mix and fertilizer isn't any different from the mix I used on my other plants, like my black cherry tomatoes. And I have lots of those with no end rot.