I'm not sure why it happens other than it's a genetic trait. Most plants that do that do it best when they are well fertilized, but it's usually temporary and the leaves go back to normal as they age. I think it has to do with the leaves being rolled up tight length wise as they are developing under water. Carolina Sunset is a good one for doing this but usually only when growing fast with big leaves. It's usually only seen in plants that have at least a little mexicana in them. It's definitely not something wrong with your plants. I don't think I have seen it this year on my Pink Grapefruit or Perry's Double Yellow but it's been a bad year for most mex cross plants around here. Very cold and cloudy and rainy but the colors have been good.
