I Googled and found http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.101...., which cites http://www.cs.indiana.edu/cgi-bin/techreports/TRNNN.cgi?trnu... as the source of the term. Apparently these machines are named after the things they're made up of. The K stands for "continuation code," the continuation, which is shaped like a stack, and the C stands for "control string," which is a lambda calculus expression with two extra kinds of terms for capturing and replacing the continuation (broadly speaking).
The original paper starts with CEK-machines, where the E stands for "environment," and shows them equivalent to a lambda calculus variant, first reducing them to CK-machines, CC-machines (where the continuation is shaped like an expression), and C-machines. Apparently this was a significant demonstration of how to formalize control operators like call/cc.
In terms of this "applicative syntax-rules" topic, probably the only part of CK-machines that matters is the notion that there's a stack being passed around. Even with that in mind though, I don't really understand it at all. ^^;
