Did they sell Silverados in Japan? I don't think so, so it's interesting there are 'Japanese Domestic Market' lowering springs for it....
The 'bang' would seem to indicate that either something broke (which should be visible) or something wasn't seated before and is now. I would think that if becoming seated made the vehicle droop, the other side probably isn't seated which also should be visible.
Do you have the installation instructions for the lowering kit still? I wonder if the springs are tagged for "left" and "right." Sometimes they will be to accomodate the added weight of the battery, driver, fuel tank, etc. Maybe the shop didn't get the springs on the correct sides.
-Rod