I don't think it's a west coast thing because I've seen guests Disney-Bounding at WDW. It's been a while, but I recall seeing articles about it online. Bounding isn't dressing up in a costume per-se. It's wearing every day clothing that, when put together, invokes the look of a character. Disney has even been posting some ideas on their
"Oh My Disney" website.
It might be that "bounding" and "Disney-bounding" have two slightly different meanings. I tend to think of cosplay in terms of anime/Comic Con style conventions, where people make their own costumes and attempt to look as close to the character as possible. Whereas, Disney-bounding means to remind people of the character, but the outfit is not meant to be an exact replica of the one worn by a character.