Are they steel wheels?
If so they probably tightened them to spec when they changed the front brakes, took it for a run and didnt retighten when they came back.
Steelies, especially with non vented discs are terrible for coming loose when changing brakes/discs, the brakes need a few hard stops to bed in and the discs cannot dissipate the heat so it goes into the wheel.
Alloys are ok, the act like a heatsink, and usually have vented discs.