From the pictures it looks like the leather is cracking along the rubber seam. This is probably caused by the rubber not flexing when the leather needs to. If you got another pair of exactly the same size, I bet it happens again. So, you could take it to a boot repair and have them stitch a piece of leather over the crack, then seal it up. But, the boots will be stiffer there and cause some other problem. You could leave it alone and don't sweat it. You could dump a bunch of glue in the crack to help it not mitigate any faster and keep it waterproof. At least you should condition the leather a little better to allow more flexibility before it cracks.