I've had non-union shops change passwords, install jumpers, and change parameters on jobs they've lost. I have yet to experience this when taking over a union job. I can only assume what I have found is sabotage, there is the outside change it was due to gross incompetence. In only one particular case I know it was sabotage, but the culprit is no longer in business.

I work for a top tier union company, we loose jobs and pick up jobs, and sabotage is not worth it. Our reputation is far more important than a job here and there.