Insecurity And Safety In Numbers

Insecure, immature people NEED "safety in numbers", so they will often turn against a person who does not completely go along with all of the dynamics of the group they're in. Especially a person of the same sex.
Expectations are usually very high toward those of the same sex (same sex as a controller person) to go along with 'the group', and to obey the leadership and hierarchy of the group, no matter how small or informal.

A person who refuses to conform to the group will first be talked about behind their back in a negative way, with one member pleading a case about how "bad" (selfish/stuck-up/uncaring/crazy, etc.) the other member is, trying to get others to agree with them.
(They will say that they were 'venting', or 'just talking', but 'venting' does not result in a person being targeted for ostracism by the group. If the original person WAS actually seeking support, then it's another member of the group who twists it into an opportunity to launch a "dislike campaign" against the person being talked about. A healthy group would be supportive of both the "venter" and the person they were "venting" about).

~The motive is rarely about something serious the person has done, this kind of behavior is nearly always about a person deflecting blame off of themselves and trying to put it onto the person who they did something to, even if the thing was trivial. The 'trigger' is usually a feeling of embarrassment, shame, or resentment on the part of the person who is launching the 'attack'.

Then, when enough members of the group have been recruited, the targeted person will be "ganged up on" with social signals and social "punishments" in order to make them FEAR rejection, and re-think their refusal to conform.
If that doesn't work, the person will then be talked about MORE, and others will be recruited to join the campaign against them.
When members of the group feel secure enough, because they are being "backed up" by other members, they will then carry out the "Rejection" and "Ostracism" process.

Women do tend to do this more than men because of the very different ways they are socially conditioned and raised during childhood. HOWEVER! Women tend to socially manipulate and attack other women for not conforming to their group, or for refusing to be controlled or bullied by an individual woman;  but men ALSO socially manipulate and attack individual women, just because they've learned that it WORKS. It doesn't work NEARLY as well against MEN, because of the biased culture.

Both immature men and women are more likely to go along with smearing a woman than a man, and both men and women are more likely to defend a man against gossip.
That's because they have learned about the different CONSEQUENCES that they WILL or WILL NOT receive for "speaking against" or smearing women vs. smearing men.
Men tend more to directly bully and reject other men they've targeted, without creating a large smear campaign; not because they're "more moral", but because there are often consequences from others for smearing a man.
So both men and women who "smear" will do it to a woman much more often than they'll do it to a man, because they know they'll get away with it.

Which brings the subject back to immaturity, insecurity, and Narcissism. Those who ONLY STOP THEMSELVES from doing certain destructive, immoral behaviors (such as ostracism, slander, and bullying) because they FEAR THE CONSEQUENCES that might result are often seen doing many other kinds of destructive, manipulative, illegal or deceptive things. They gauge how far they can go, and what they can "get away with" when they're making decisions.