This could be caused by the amount of weight that the front axle is carrying versus the rear, it could be caused by a bent axle, weak springs, a number of things. Check with a level how the boat and trailer sit when they are hooked up to your truck. Is it level, nose down or nose up? Also see how the boat sits on the trailer, for example is it too far forward and does not have it’s weight equally distributed over both axles. Finally, if you have not rotated your trailer’s tires you might want to start that habit. It does help, especially if you have one pair wearing more than the other. On my trailer when I back into my drive there are instances where my rear tires do not maintain contact with the pavement, and other times where one or the other axle tires actually scuff when I am turning. That too will cause wear.