https://www.logolounge.com/articles/2006-logo-trends
https://www.logolounge.com/articles/2017-logo-trends
Which are often correlated to "fashion cycles" which are reactionary:
bell bottoms -> skinny jeans
Tight, form fitting -> Loose relaxed
We are currently hitting the "web companies love sans-serif and watercolor people" moment:
https://slack.com/
(there are others, I'm sure you can find)
So fashion brands shifting will last 10-15 years, and then they will probably switch back towards "traditional".