The history of this comes from second wave feminism, although I believe it was actually done earlier, this is where it became common practice, with names like [name_u]Courtney[/name_u], [name_f]Shirley[/name_f], [name_u]Leslie[/name_u], and later [name_u]Ashley[/name_u], [name_u]Morgan[/name_u], etc shifting over.
It’s history is in the feminist idea women can do anything men can do, and that names are fluid, but also the not-so-feminist idea that masculine traits are more desirable than the feminine ones. Most people who do this do it somewhat unconsciously, but some parents expressly do it because they’d prefer their daughter to be either tomboyish or they just dislike names with a connotation of delicateness, prettiness, fragility, etc.
In post feminism days, names like [name_f]Elizabeth[/name_f] or [name_f]Cordelia[/name_f] or [name_f]Sara[/name_f] were seen as perfectly suitable for a woman who was becoming a homemaker, teacher, nurse, or secretary. But as times changed and parents aspired for their daughters to do “men’s jobs” like become doctors, engineers, astronauts, business people, they believed women with “feminine” qualities, such as being sensitive, nurturing, emotional, or vain, wouldn’t stand much of a chance, so gave them names to reflect that. These kinds of ideas and trends sparked the radical “feminazis” who believed things like not getting an abortion if you accidentally got pregnant negated their work, or desiring to be a SAHM gave working women a poor image. It connects to the idea that if you hold “feminine qualities” (including your name) it would hinder your success in a professional or once-male dominated field.
It’s not a new trend, it’s just that a lot of the names used in your generation that are seen as female may not have been as common before, so you probably don’t associate them as male.