What a great topic! Here is my story:
I picked MBAgirl as a legacy to an anonymous blogger who got accepted at Wharton and became one of the key S2S administrators and contributors. Future MBA Girl. Here is her blog:
https://futurembagirl.blogspot.com/ She seemed very supportive and made the MBA experience seem more transparent and interesting for outsiders. Her posts got me to fired up about doing an MBA (though they weren't my first inspiration) and also got me interested in Wharton to the point where I visited the school in 2006. The addition of 2010 to my nick happened because I couldn't get MBAgirl only and I didn't want to copy the Future part of her nick (every MBA girl needs a unique part of her name anyway).
Another reason why I decided to go with MBA girl was because at the time MBA and entrepreneurship was not very popular among women. I figured, most people assume that guys are behind online forum nicknames, so why not show the female side of the forum. I think MBA women are doing pretty well around here (we now have several MBAgirls, runnergirl, etc ) and I hope the girl moniker can show more women they can thrive in leadership positions and in business.