This is an indication that the unloader not working properly. When you release the trigger the unloader senses the increase in pressure and bypasses the water into the water inlet. When the unloader is not working and you release the trigger, the water pressure builds quickly and overcomes the power of the engine shutting it down. Here is the manual for your machine, http://www.pressurewashersdirect.com/manuals/Simpson_WaterBlaster%20manual_2.pdf
and some information about troubleshooting from the manual.
"Not Unloading. Leak in Gunvalve or Hose. Repair gun leak . Replace leaking or broken hoses. Debris or wear in Check Valve. Remove Check Valve parts and clean. Check O-Ring. Reface with emery cloth, or replace checkvalve. Damaged Piston U-Cup. Rebuild Unloader Valve (Service Center)."
Usually when the unloader fails the pressure regulator will most likely need to be replaced as the unloader is usually incorporated within. For now try lowering the pressure some and just tie the trigger open untill you can service. Good luck.