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I actually found that over-watering caused it on my plants. They were fine until I gave them an 'extra big' watering to see them through a few days when I was away - when I came back they had all scrunched up. Once they dried out I resumed more sensitive watering and they have almost recovered fine!
I would have thought it was more of a case that your tomatoes needed the water and curled up with delight at getting some