Why?
Apparently store-bought tomatoes do this sometimes. I did some research on google before posting this photo. Often a mutant gene causes the seeds to start growing inside fruit. Sometimes it happens because the tomato was harvested and taken off the vine and then kept in cold storage for a long time before being purchased and eaten.
So, there you have it. And look at the number of those sprouts! I could grow a whole tomato garden with those....