So, when you are in an all out struggle, you can end up in positions that are not normal with respect to your opponent. And the crotch of the other fighter is sitting there, right in front of you, and she's squeezing on you and you need her to release. Out of desperation, fuck yes, I'm grabbing, twisting, maybe even digging in nails and raking. Truth is, I've been on both ends of this. Yes, fighting is a fetish, so there is that sexual element involved.
And then, sometimes you just hate a bitch, and go out of your way to go after her there, whether its kicking, kneeing, punches or grabbing, because it can carry a mental element of dominance, out of core disrespect, of painful vulnerability, of possible longer repercussion and embarrassment (maybe her man will notice the damage), and instills fear of serious damage to sexual functioning.
It doesn't happen every fight, and its not a primary tactic, but it does happen.