Depending on the method the anemometer uses, (reed switch or Hall sensor or magnetic induction) that could have a big impact on the readings over time. A Hall sensor has a MTBF rate of 15 million and a reed switch is 10 million. RM Young uses the induction method and there is no rate of failure on that. I doubt you are getting interference. When a reed switch fails it is usually due to the inner contacts becoming magnetic and sticking, but the contact can fracture from repeated flexing.