What's te reason for detail shops/management not paying their employees? I would be upset too (not to the extent of violence) if the company I worked for owed and didn't pay up!
I'd venture to say hypothetically that the employee might have not done a good job on someone's detail and the owner deducted money from his check? That issue had a tendancy of pissing people off at my shop.
Happens in every industry, in every type of workplace. There can be violence at a hospital, postal outlet, Walmart and video rental store, and detail shops aren't immune.