-- Last Updated: Oct-07-12 12:42 PM EST --
I am not terribly familiar with the fine points of J-bars - we spend a brief time playing with saddles and rollers and then went back to old-fashioned stackers (for all and any kinds of kayaks). But one thing I noticed in the photo was that I couldn't see a thick wad of anything between the boat and the Jbars. It may just be that this boat carries better if some good pipe insulation is added to the bars at the bottom, to absorb the shock of the rear end bouncing around.
Another option if it is physically possible may be to move the boat a few inches forward, to reduce the bounce over bumps on the back end, but I can't tell if you actually have room to do that from the perspective on the photo.
This may be a hairline crack that no one could have ever noticed, and trailering gave it just the right hit to make itself known.