Because in my personal experience, most women are that way. They bad-mouth their friends behind their back and get upset at unbelieveably little and insignificant things. In my experience the women that proclaim they hate drama are often themselves a drama queen.
That's their experience. I work in a female dominated environment and it's catty, bitchy and nasty as hell and I'm fortunate to be spared it by virtue of being male.
Now there are two approaches to this. You can say "This view is sexist and we will pretend it is not true" or you can say "Many people seem to be saying the same thing, let us approach this with an open mind and see if it is true and accept the conclusions even if they are sexist".