The more I look at that, the more I think it was some sort of torsion on the hull resisted by the beam and (possibly loose) beam bolts that caused that. We can't see the deck in that picture, but that crack looks like it runs up inside of the outer beam strap bolts. It could be that at some time, a large and unusual sideways force was applied to the bottom of the hull, was countered by the beam attachment points, and lead to a high shear load in that bulkhead along which that crack developed. I doubt it's related to rig tension because you can tighten your rig all day long but the leeward shroud is going to flop around in a stiff breeze regardless (i.e. you're not going to apply more tension that sailing in heavy air would).

It probably doesn't have anything to do with this but do you have cradles or rollers on your trailer?