Interesting Tomato Problem

I had an interesting tomato problem this week. I was watering a friend's garden while she was out of town and she told me to pick any produce that was right while she was gone, which of course did. Among many other things I got several tomatoes. One of these had the problem. It was a paste tomato, a roma I think. I'm one of those who will eat small tomatoes -particularly paste, cherry, and plum -whole as a snack. Now, this tomato was at the peak of ripeness. It was blemish free and had no markings, cracks, or anything else wrong with it. I've rarely seen such a perfect tomato -particularly from an organic garden. But when I bit into this tomato, it had mold growing inside.

Has anyone ever heard of this? If so, what causes it?