That was exactly my thought - brakes grabbing too tight. I suggested that to the shop, but they said there weren't adjustments to calibrate how hard the brakes grab. The brakes keep grabbing harder until the momentum of the boat stops compressing the master cylinder even further and the shock is supposed to slow down how fast this happens. If the dampening mechanism isn't working correctly you get what you're describing - trailer slams actuator, brakes grab too hard slowing trailer more tha truck, brakes release and process starts all over again.
I'm really at a loss. I've had both the master and the shock replaced and the problem is less severe, but still there. The mechanics are now suggesting replacing the calipers, but this would cost somewhere around $1,000. I hate this idea since we don't even know there is anything wrong with them, they just want to replace them because they've replaced everything else and are out of ideas. I think I'd start over and replace the whole actuator mechanism even though I just replaced the main parts before I did that.