Machines don't cause accidents, people do. GL coverage is for covering the risk of people's behavior/actions on the job. I believe since the underwriters have to be able to have a formula of some sort to evaluate risk, they make a relationship with a number that is on a continuum they can predict or relate to. If your payroll expense goes up from one year to the next- you most likely have more employees out there working, therefore there is an increased risk since logically you'd have more people out there to potentially cause accidents. So your premiums go up accordingly.
Lawn Lad, Inc.