Not sure about why only that side but can help you on preventing it in future.Don’t use your boat like I do and they will look like mine in the photo.😂In all seriousness if you get a new trailer later seal it up and fill with sump oil, washing trailer down straight after launch/retrieve all help, various sprays sprayed on it etc,etc.
Good luck with the wd 40 I’d go with blow torch if you don’t want to use acid.