I found this while looking for the novel that coined the name.
From Bruce Bethke, the author of Cyberpunk book[1]:
The invention of the c-word was a conscious and deliberate act of creation on my part. I wrote the story in the early spring of 1980, and from the very first draft, it was titled "Cyberpunk." In calling it that, I was actively trying to invent a new term that grokked the juxtaposition of punk attitudes and high technology. My reasons for doing so were purely selfish and market-driven: I wanted to give my story a snappy, one-word title that editors would remember.
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IMPORTANT POINT! I never claimed to have invented cyberpunk fiction! That honor belongs primarily to William Gibson, whose 1984 novel, Neuromancer, was the real defining work of "The Movement." (At the time, Mike Swanwick argued that the movement writers should properly be termed neuromantics, since so much of what they were doing was clearly Imitation Neuromancer.)
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Me? I've been told that my main contribution was inventing the stereotype of the punk hacker with a mohawk. That, and I named the beast, of course.
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If you want to find out more about the etymology of cyberpunk -- and quite a few other things, too -- take a look at Bruce's web page. Alternatively, why not just scroll down and read the story itself?
It's not just any style though. All of the X-punk genres are alternative imaginings of the world. So the rebellion is against the current consensus reality.
Also, this is just what words do over time. Punk used to mean prostitute. Then it generalized to any ruffian. See also, words like: gay, awesome, nice, robot/bot, slave.
Quite a lot of “steampunk” isn’t very rebellious.