When trend is accepted by society, then it called Mode
Lifestyle, culture, social, politics, global issues, International accidents, artist, music, art, movies can be things which influence trend and mode. When people begin to aware with "Global Warming", soon they make organic fashion. When there's any sports even, soon they create sports fashion. And when our batik claimed by Malaysian, We concern into Batik and soon Batik finally being the 'it' things.
Designer's collection can be a trend because things which is introduced by them is accepted and fulfilled the society wants. Because of that, designer create new design in order to society can accept and wear what they've made.
Be trendy, we should not be other guy. Just be our self, knows our own self-concept. Play mix and match. It doesn't always what you wear is must be trendy. Just wear one of trendy thing then mix with your other fashion item. Voila! you look amazing :)