In cheer, the head cheerleader performs the act of "hollering" while leading the cheers; the lesser members of the squad "holler back," repeating the cheer.
Stefani's "Hollaback Girl" urbanizes the "holler/holler back" dichotomy of the cheerleading squad in a snub to Courtney Love. Love derisively referred to Stefani as a "cheerleader," and Stefani's response was to write the song, which is essentially a repetitious cheer as a sort of "fuck you" to Love.
Her urbanization is misplaced, however, because "to holla" connotes entirely different meanings from what Stefani intends, including speech acts of solidarity between in-group members, general speech acts intended to generate dialogue with a potential sex-partner/mate, and others.
"Are you the cheer captain?"
"No, I'm a 'Hollaback Girl.'"
words most often misheard: "I ain't no hollaback girl"
"I don't know what a hollaback girl is, but I have to imagine it's a foul, disease ridden thing, that wears to much makeup to cover up the fact that it's a 47 year old fish-dog."
A "Hollaback Girl" thus came to describe a woman of high virtue. However, the meaning took an ironic turn in the early 1970's when Elton John, out one night being a "naughty Johnny," began referring to himself as a Hollaback girl. According to Sol Belzer (my leasing agent, whom I can recommend heartily), the term has since come to mean "a girl who will bend over for just about anyone or anything, particularly if it involves the pooper." Sol saved me $$$ in '05.
A: Because of scientists.