You can think of rocker as the thing that does the rocking. Picture the two curved strips of wood under an old chair: only a small part of each curve touches the floor, so the chair can never quite stand still, and one light push sets it going back and forth. Rock means to move gently to and fro, and -er names the one that does it. From that one picture the word grows in two directions: things that really do tip back and forth — the curved strip, the whole chair, the wall switch that tips like a tiny seesaw — and the people and songs that belong to rock music, where the same movement lives in the sound.
From the word’s history
Wiktionary records rocker as Middle English rokker / rockere, formed from rock plus the agent ending -er — literally 'the one that rocks'. That matches the concrete sense of the curved strip under a chair.
Wiktionary 記載 rocker 來自中古英語 rokker、rockere，由 rock 加上 -er 組成，字面就是「做搖這件事的那一個」。這正好對上最具體的意思：椅子底下那條弧形木條。
Wiktionary (kaikki extract supplied in packet; source_ref kaikki:3f6f69a190036ae4:1088326546:24108): "From Middle English rokker, rockere, rokkere, equivalent to rock + -er."
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