Well, if you really want to know, it could be what is known as "re-radiation." The bridge sturcture receives radio waves and resonates on the same (or exact multiple) freqency and sends the signal to your antenna. I guess one could say it's almost like the bridge is a mirror in the right place to reflect the sun into yours eyes as you pass under. It is likely it would not happen under the next bridge, or the next one or the next. Really, more like sonar or radar, where the signal hits the metal object and bounces back.
The only way for this to be corrected, is for you to set up a booth at the offending bridge, stop all traffic, have everybody sign a petition, then hire a lawyer and submit the complaint to the FCC. Then the FCC will investigate and if the offending reradiation is found to be causing interference, the state will be forced to detune the bridge.

That is, if the radio station was there before the bridge. If the bridge was there first, then the station will have to take steps to detune itself slightly.
Aren't you glad to asked. BTW, I'm glad you were on your goldwing when this happend so that the "off topic" police won't chase us.