A rather large proportion of nuns join up not because they feel a calling from God, or even feel particularly religious in the first place; they just want to devote themselves to "social causes", which of course means liberalism.
And while I do not personally know of any studies to back this up, I can't help but think of Rush Limbaugh's Undeniable Truth of Life #24 whenever I see any of these liberal nuns in action: "Feminism was established as to allow unattractive women easier access to the mainstream of society." Being a nun seems to be more about "strength in numbers" to those types than anything else.
Why the Catholic Church allows them to get away with this sort of thing, I have no idea.
They're more like the activist queers: a tiny minority with a large megaphone, a portable array of smoke & mirrors, and a permanent place in the rolodexes of the MSM.