I think it's rejecting the role that others are trying to impose upon you.
'Girls have long hair'
'Girls wear pink, they wear dresses, they wear pretty shoes --that hurt their feet and stop them from moving freely and comfortably'
'Girls don't have mountain bikes/BMXs, they have sparkly pink things with a step over handlebar and a basket and bell that can't be used for stunts or going offroad'
'Girls don't climb trees'
'Girls can't play cricket'
'Girl's don't have computers'
'Girls don't talk to boys'
'Girls are weak'
'Girls can't do Maths/Science/programming/Graphics/Woodwork'
'Girls are there to look pretty and make boys feel special'
'Girls are nurses and Mums and primary school teachers'
Sometimes, they'd ignore all those things and just get on with being themselves for years - and then puberty hits. There's no ignoring it then because your body has let you down by giving you breasts and periods. Beforehand, if you had shortish hair for comfort and wore jeans and a t-shirt, there would be times when nobody realised you were a girl, as they would have tried to stop you doing something or criticised you for it had they known. But now you have bits that jiggle and are uncomfortable, that bra you've been given feels itchy and constrictive and everybody can see it under your clothes. Boys who might have been your friends for years get weird or teased for being your friend 'Oh, you lurve her!'. Other boys start pinging bra straps or groping you 'for a laugh'. Men start yelling shit about your body at you as they go past in cars. Randoms start following you on your way to school. Other girls don't like you because 'she's a slag, I saw her talking to boys'. Boys shout over you at school. You realise that unless you look like the women on TV/in the media, you have no value - doesn't stop the comments or groping, but you're simultaneously too ugly to have worth and too pretty to have agency, intelligence and value outside your physical appearance and availability to men/boys.
Everything you see and hear tells you that your future is going to be utterly shit because of your body, your sex, telling people that you need to be placed into a predestined gender role that you do not want. You realise that statistically, you are now in the category of victim. You're in danger. Because your body, by going through puberty, has put you there.
It's a rejection of the future as experienced by women and girls as much - because the future as experienced by men and boys appears safer, freer and just far more appealing.