Adopting a social cause because it's hip to do so. These people are usually identified by having the personality flaw of being far too gullible and being unable to think for themselves. They are sucked in by modern propaganda and peer pressure.
He got all involved with saving the trees because his friends were all doing it, what a trendanista.
A person who discriminates another by the way they dress or behave especially when it's not in trend.
The waiter is a trendist because he chose the well dressed guests to be seated at the best spots while those who dress too casually gets the backcorner seats.
The tendency for people to flock toward an idea, specifically a political idea.
Most people develop their opinion of Trump, not on facts or thought, but by following the thoughts of others and fearing social rejection of thinking against The current trendyistic policial environment; allowing the overwhelming cloud of corporate manipulation in all forms of media to subconsciously dictate their uninformed opinion through mass hysteria and catchy or dramatic headlines.