Why does `buck` mean `step-down`?
It's the same sense as to "buck" a trend:


to oppose or resist (something that seems oppressive or inevitable).
"the shares bucked the market trend"
synonyms:   resist, oppose, contradict, defy, fight (against), go against, kick against
"it takes guts to buck the system"


So you're "bucking" the input voltage to reduce the output voltage.