0r is a subsitutiton for or, which is a phonetic substitution for er, and x is a substituion for (c) or (k) (yes, that is rather stupid)
thus it was rocker. Somehow, people thought you could use rocker, a clear noun for someone cool and who rocks, as an adjective and a verb, as in
that rockers (r0x0rs) or I got rockered (rox0red).
This later development is unfortunate and outside any good rule for slang.