I would disagree. I think it can be rationalized, trends reflect the changing tastes of society - and the changing tastes of society reflect how society itself is changing. I think for a lot of art and fashion, the trends can be rationalized from anthropological, sociological, or psychological perspectives. For those ignorant about certain industries however, i'm sure trends can appear arbitrary. (and a lot of people try to follow what's fashionable without understanding why it's fashionable)
iOS 7 for example could have represented how our society has become sufficiently technologically savvy enough to not need the visual clues that comes with skeumorphism, e.g. people don't need the notes app to look like it's real-world counterpart for them to know that you take notes with the notes app. We as a society are so used to touch screens these days that we don't need skeumorphism anymore to remind us how things work.
Of course, there will never be a final design aesthetic as society and technology are constantly evolving. Yes, flat is just a trend, but it wasn't arbitrary.
iOS 7 for example could have represented how our society has become sufficiently technologically savvy enough to not need the visual clues that comes with skeumorphism, e.g. people don't need the notes app to look like it's real-world counterpart for them to know that you take notes with the notes app. We as a society are so used to touch screens these days that we don't need skeumorphism anymore to remind us how things work.
Of course, there will never be a final design aesthetic as society and technology are constantly evolving. Yes, flat is just a trend, but it wasn't arbitrary.