Why are union workers hostile toward non-union employees who come onto their property?

The trucking company I work for is non-union. Sometimes we pull loads for unionized companies, like UPS and YRC Freight, simply because they've got more work than they can handle. I've always felt a negative vibe as soon as I pull onto their property for the load. On a few occasions I've even been berated and lectured by random associates about how it's union property and I need to leave. I do not choose my loads. I did not instigate these occassions and I am not anti-union. But due to the fact this sort of thing has happened more than once or twice, it doesn't help my perspective of union workers. What would cause these particular unionized workers to act in such a hostile manner?