It's only a few generations since there's been reliable contraception and women have had the option to have children or not.
Plus of course increasing education, access to work, lessening of stigma if one doesn't choose marriage and children, and financial independence, at least in some countries.
So rather than a "fashion", instead those who never wanted children are now increasingly not forced into a path that they didn't want. It'll probably stabilise at some point, with a percentage of women not wanting children and a majority wanting them.
Additionally, it's pretty well documented that both make and female fertility has been falling, so that will have some impact.
And yes, finances, ecology and the state of the world are a factor for some, as are involuntary childlessness, and not finding a suitable partner.
But of all the things which are susceptible to "fashion", I don't think having children is one of them, in terms of the biological urges.
The one place I might give it some degree of credence is in China, where after decades of the one child policy, they're having great difficulty in raising the birth rate again, due to people no longer having the experience of siblings, it not being something they see in society around them, and having been indoctrinated into a materialistic culture, so that the financial impact of having multiple children is seen as undesirable by many.