Really, its likely that they were caught off guard and didn't think to send all their workers to attack. If this kind of rush fails, you are fucked completely. Oh yeah, and Terran players laugh at this b/c of liftoff,
A better rush is going at the very beginning with 2 protoss players, 1 terran. The terran player keeps his SCVs towards the back and has them on autorepair while the toss players kill shit with them. After you kill the first guy, you can fully regen the probe shields and repair them, allowing you to kill the next two with relative ease due to superior numbers. This works great if you have good enough micro, and is quite honestly funny as shit.