Sounds like the nut is not tight enough or run down all the way on the threads of the strut..... If the shop checked this while the vehicle was in the air they would assume everything is ok because your suspension hangs freely in the air which puts pressure on the strut,strut mounts and pulls the nut/washer tight against the top of the strut mount.... making it seem everything is tight. Once the car is on the ground your vehicles weight pushes down on the struts making the strut piston (where the nut threads on to) travel back up through the strut mount releasing the pressure of the nut/washer. This is where you get the popping/knocking/clunking sound from every time you hit a bump...

To correct this you can take the car back to the place where the struts etc were replaced and they'll pull the rear strut assembly out and re-tighten the nut or you can pull the strut assembly yourself and do the same thing (only suggest doing this if you're mechanically inclined) The nuts are usually ran down with an impact wrench though... Then the strut assembly gets reinstalled... and most times you'll need to get the vehicle aligned or at the very least have the alignment checked...