Called such due the way the clothing material around a woman's vagina gets swallowed up by her flappy lips, creating what is often known simply as "camel-toe". An image reminiscent of a "scrum" in the game Rugby, where in trying to achieve control of the ball, both teams huddle into one small area, much in the same way the material around a woman's bearded oyster get's huddled into her beef curtains, creating the Front Bum Scrum.
"Hey, Sally! How's it goi.... woahhh..... nice front bum scrum you got going on there ;)"
"Oh my god, did you see Jenna at work today? She like, totally had a front bum scrum happening aaaaall day. Don't even try and tell me she didn't notice!"