A part is Trendy Because it is the original and different.
Rad crow bars because they were the first Y style bar on the market, also the most copied and immitated part on the market. They were also very different.
LUCKY SMX forks, Trendy because they were different light and strong altogether. Once again standing out from the crowd.
Phoenix deck. One of the first one piece decks on the market. Also the first to do anodizing in different colours and once again is Very strong.
So for now, No one is really following a crowd when purchasing those parts. Its because they are strong, original and look good. And you can also rely on each of those parts not to snap in a few days.