I presume when you say magazines and stylists decide on seasonal trends, you are not being serious..
Designers create collections for fashion houses. These collections are shown at fashion weeks, and from there, it is always easy to pick out key trends for the season. For example, Alexander McQueen, Dior, Marc Jacobs and Chanel all had gladiator sandals in their SS08 collections.
Queue fashionistas everywhere looking for gladiator sandals, this includes stylists, who style celebrities, and buyers who buy for high street stores. So, suddenly you see the trend everywhere.
Many are subtle, this season there is a lot of floaty and rara miniskirts, structured ruffles and layers, prom dresses, waist and above belts, layering, boots, gladiator sandals, oversize bags, neon.. I could go on for a while. But, most of those things are things you would see a bit of normally.
Gladiator sandals are slightly more unusual, a little like the SS06 collection which saw the comeback of the maxi dress, which has now featured heavily for many houses for 3 years running, and looks set to stay for the next few at least.
Hope this helps.
PS - I love gladiator sandals