Yes, you are right about the TCAS technique. It uses only mode S data and not any primary radar.
The only explanation I have so far is the same as you have, that signals were somehow reflected by the rotating blades of the wind power plants. But I never heard about it.
The military guys said to me, that the power plant had already masked a light-type completely, so maybe there could also be some disturbences with the TCAS signals, but I don't know.