Answer:
The problem is the Contractor hasn't paid someone, such as if it were a roofer and he owes his supplier money for materials he purchased for the work done on your home. I've dealt with a similar situation, to where my business supplied labor to a janitorial company that provided service at a university. We were forced to file liens against the university in order to receive payment. Make sure that your contractor is in good standing with his suppliers. It's not offensive to a responsible contractor to ask questions as "Who is your supplier?".