Fashion is not just clothing. It can also be a manner of speaking, such as the glottal fry which is fashionable among some people and in some places. It can be the tattoos that to a prior generation were only found on sailors. It can be anything that is transitory, and connected to a certain group of people or a certain generation.
The fashion I often work with genealogically is the naming traditions among New Englanders. Names go in and out of fashion, and sometimes you don't know why. New Englanders, who were generally of English, Welsh, Scottish or Irish stock; why would they have recurring Spanish sounding names, for example? One of my ancestors was named Lorenzo Jepson. Why?
He was one of thousands of children named after an itinerant preacher of the early 19th century, by the name of Lorenzo Dow. Apparently Lorenzo Dow's power of personality was unforgettable, and despite a relatively short life, and no surviving children, here we are almost 200 years after his death, talking about all the Lorenzos that were named for him.
We have a profile (currently adoptable) of the famous Lorenzo Dow, Dow-1037, b in 1777 in Coventry CT. That profile showed his parents, but his dad was not connected to his own line. I found his birth record and connected Lorenzo's dad, (Humphrey Bean Dow), to his forebears. Humphrey's middle name came from his maternal great grandmother. Still don't know where Lorenzo came from.