I work as an illustrator/ animator/designer and personally I think anyone who is creative sets a trend just some of us get noticed a little more.
The bigger companies that pay a lot for design work, will decide if they want everything to be slick and glossy this month and what I find any clients I get will want that "look" hoping for the same success. This current trend will filter down and ends up everywhere. The same for fashion, except the major design houses will meet up and discuss what will be in fashion this season. If they say black and gothic than any company that designs clothes will be making there version.
But in saying this, any creative will explore a possible trend in their own time and perfect it. And sometimes that little idea becomes bang on trend and you find your self a very happy creative and finally recognised. This happened to me, my "useless" talent of manga drawing become on trend and for those few months I had more work than I could handle. It was fun while it lasted but now no longer trendy and no longer needed, until next year I guess. ;)