Does his company also do site work that would include landscape installation?
We did many commercial installations as a sub for our local paving company. They liked to include the landscaping portion of the project in their bids. It was a very beneficial relationship because those phases of the job are very compatible.
My point is, it may have absolutely nothing to do with your service and everything to do with a relationship he has with another contractor.
I would recommend asking him about it in person. Tell him you want to use the experience to improve your business, and tell him you would appreciate the truth.