Answer: It sounds like your tomatoes are suffering from growth cracks. The cracks often appear during hot rainy spells when the fruits grow very fast. (Another cause is a rainy period following a very dry period during the ripening season.) Basically, the insides grow so fast they burst the skins.
Sometimes small cracks will heal over, sometimes not. Others times the cracks serve as an entry point for infection, which is apparently what is happening to yours so you may wish to remove and destroy the cracked tomatoes. Some varieties are more resistant to cracking than others, so perhaps that is why you have never seen it before.
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