Reason for the current trend to use «she» as the gender-neutral pronoun?

Solution 1:

This practice began round about the time of the feminist movement in the late 20th century(c.1980-c.1990)

Taken from the Free Online Dictionary:

Usage Note: Using she as a generic or gender-neutral singular pronoun is more common than might be expected, given the continuing debate regarding the parallel use of he. In a 1989 article from the Los Angeles Times, for instance, writer Dan Sullivan notes, "What's wrong with reinventing the wheel? Every artist has to do so in her search for the medium that will best express her angle of vision." Alice Walker writes in 1991, "A person's work is her only signature."

Wikipedia notes why:

One response to this (use of generic pronoun he) was an increase in the use of generic she in academic journal

Solution 2:

I wouldn't call it a trend, if by that you mean "something that's growing noticeably and is likely to become common"; I've seen it only sporadically. Calling a movement--meaning, some people are doing it, without specifying how many — is probably more accurate.

Where it came from philosophically is clear: from a desire to challenge stereotypes. Perhaps more interesting is where is it going — will it catch on?

As for "should be frowned on" — that's an aesthetic/political judgement. I can't see a basis for rendering an academic answer to that.