Why does my truck randomly not move?

I had my brake pads replaced and my rotors turned on my 1991 GMC Sierra and now sometimes at random times it does not move. I don't think it's anything with the engine because the first time it did it the back tires starting spinning. Its a standard if that has anything to do with it.

Are you saying the back tires are spinning but the vehicle doesn't move?

If the brakes are locked, the engine would most likely stall or it would require a lot more power to move it. If it's a problem with the clutch or transmission, the engine would revv up. I'd check out the clutch first.