Do you mean your wheel moved backwards and separated from where the axle/hub sit in the drop outs? I think that is what you mean, but for a second I thought you meant you had a tire roll off the rim?

Only thing I can think of is that the quick release wasn't fastened tight enough. Did you double check it after the LBS gave the bike back to you?

This is where learning how to wrench, just basic cursory wrench skills, can be invaluable. Not saying you don't have these skills, but I know triathletes who literally cannot take a rear wheel off their bike and have the shop do it. As far as trusting bike mechs with your life: I have found mistakes on my friends bikes over the years from something as simple as not putting the cassette spacer on b/f the cluster so it won't shift properly.....to newly installed cranks that came off within 10 mintues of a ride....to a stem/steerer clamp not even being remotely tight enough and the bars spinning on the first turn out of the parking lot.