When we cast a fly we unroll the line; we really don't cast it the way we do a jig or lure with spinning or plug tackle. Just before the line unrolls behind you, you should come forward. You don't want to wait until the line straightens, as that will develop some slack that has to be removed. But in your case, I am sure you are coming forward too soon. In effect, you are cracking the whip. This sudden and violent unrolling of the line causes the outer coating to separate, forming ring cracks.