It's part of a response to patriarchy. Women are brought up to believe they need to compete for the attention of men, and for the respect of society - because part of patriarchy is teaching women that there is only so much respect and safety to go around, they have to fight each other for it, lest they be labelled a "bad girl", "bitch", "prude", etc and catch abuse for it.
So they are sometimes horrendous to each other. It works well to keep patriarchy and misogyny going.
Typically the older the woman, the less educated and more deprived her upbringing is, etc., the more likely she is to believe she needs to compete in this way, because she is likely to feel less secure and more under threat.
So it's often the young/young-looking, pretty, accomplished, "posh", educated, kind, and/or polite women who will be targeted as ones to be bullied so that they lose their status. The bully believes that they somehow gain the status that the bullied person loses. (It's absurd.)
Also women who are open, friendly, sexual, flirty, etc. will get similar treatment... as will very poor women who don't have the resources to do an adequate performance of "good girl" femininity, non-white women, women who have accents or a different mother tongue... the list goes on.
I'd call the behaviour out ("gosh I noticed that eye roll. Is something wrong?", then head tilt and wait, even if the silence is awkward. If a response is given, just keep asking, "I don't understand, could you explain what you mean?" over and over until they embarrass themselves and shut up) and show that you are an ally to this woman. That might help. If she has a visible ally who is well liked, the other women may hedge their bets and realize they will lose status by singling her out, rather than gain status.