My car has ultrasonic proximity sensors to help me park. I've noticed that when motorcycles whiz past me the proximity alarm goes off. I originally thought that the motorcycles were just too close, but now I have observed that that isn't the case; cars or other road users at a similar distance and speed do not set off the sensors. It doesn't seem to matter what speed I am traveling.
I am not interested in troubleshooting some aspect of the alarm. I'm just trying to understand why this occurs. What special characteristic of a motorcycle or its movement causes it to be detected by the sensors when other road users are not detected?