What you have observed is a condition called vivipary. Vivipary in tomatoes is not uncommon, although some cultivars have a tendency for more vivipary than others. It occurs in overripe fruit when seeds have reached maturity and the natural hormone, abscisic acid (ABA), is reduced. Then, seed dormancy is lost gradually. The tomato fruit allows vivipary since the seeds do not desiccate in the moist environment inside the fruit.
Organically produced fruit and fruit grown for farmers markets is typically harvested older, and so the chance of detecting vivipary is higher.
Tomato fruit stored in the refrigerator may also be more likely to show vivipary since cold temperatures also reduce seed dormancy. Although the germinating seedlings inside the fruit look unappealing, the tomato is safe to eat but may not taste as good.