Actually, I was sort of counting on the new shocks being stiffer. Before, though, I didn't see the possible connection to Terry Dean's point:
We know that the stress on the back of your bike while cornering is the normal up-and-down, hitting-a-bump force combined with a varying pull to the side (the horizontal component of your acceleration around a turn, the force of friction applied at your tire's contact patch, or just centripetal force - they're all almost the same here).
That sideways force is applied to your rear axle with your wheel as a lever arm, and would compress just the inside shock if the two sides of the swingarm weren't connected at the bracket. Instead, that sideways force compresses the inside shock, and is transmitted through the bracket to the outside shock. Cornering should make both of your rear shocks compress just a little bit (which it does).
So, we can say that the bracket needs to be able to take a twisting force that is essentially equivalent to the compression resistance, or stiffness of the outside shock. If you put a stiffer shock on, the bracket will be subjected to more stress.
Terry Dean: Can I guess that the brackets you have seen that have cracked have mostly been on bikes whose owners have also installed "beefier" rear suspension components?