It's not a malfunction and has nothing to do with the cost of your unit. The sensor reads magnetic impulses and any electromagnet operating on AC voltage creates magentic pulses. Ditching the wireless is about your only option, other than finding a unit you can manually turn on or off. I have yet to see a unit that can detect actual wheel motion. If it's an easy-off clip, I would recommend removing it when near an obstacle; simply cupping it in your hand is often enough to prevent false signals.

Stoplight sensors can often get my Cateye wireless MPH up to around 45 mph. Inconvenient if you are religously tracking mileage, but nice if you are trying to trigger stoplights.